<?xml version="1.0" encoding="UTF-8"?>

<!--XBRL Document Created with Wdesk from Workiva-->
<!--Copyright 2022 Workiva-->
<!--r:ed3e6092-8757-4d56-8210-30b00c5ca124,g:7bc22eae-5a74-4eec-8abb-6b9bd838edb6-->
<link:linkbase xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd">
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain"/>
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:type="simple" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member"/>
  <link:roleRef roleURI="http://www.fortinet.com/role/ConsolidatedStatementsofIncome" xlink:type="simple" xlink:href="ftnt-20211231.xsd#ConsolidatedStatementsofIncome"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/ConsolidatedStatementsofIncome" xlink:type="extended" id="i75262d17c41440dbaf63ab220c51ce9b_ConsolidatedStatementsofIncome">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_ce4cd364-8145-41d0-811b-4e7df982313b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerAbstract_58c09623-375f-4038-8430-4240f9ed662f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueFromContractWithCustomerAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_ce4cd364-8145-41d0-811b-4e7df982313b" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerAbstract_58c09623-375f-4038-8430-4240f9ed662f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_834a6c90-ee22-415d-b496-241d446a69e1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueFromContractWithCustomerAbstract_58c09623-375f-4038-8430-4240f9ed662f" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_834a6c90-ee22-415d-b496-241d446a69e1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServicesSoldAbstract_1d2ad519-0e48-4b59-9edd-79c12a2e7fdb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CostOfGoodsAndServicesSoldAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_ce4cd364-8145-41d0-811b-4e7df982313b" xlink:to="loc_us-gaap_CostOfGoodsAndServicesSoldAbstract_1d2ad519-0e48-4b59-9edd-79c12a2e7fdb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServicesSold_e0d5b829-599f-4d6d-ac44-790ad3bbbb59" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CostOfGoodsAndServicesSold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostOfGoodsAndServicesSoldAbstract_1d2ad519-0e48-4b59-9edd-79c12a2e7fdb" xlink:to="loc_us-gaap_CostOfGoodsAndServicesSold_e0d5b829-599f-4d6d-ac44-790ad3bbbb59" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GrossProfitAbstract_f3f1a6f1-1f13-465e-9bc8-3cb83154c314" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GrossProfitAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_ce4cd364-8145-41d0-811b-4e7df982313b" xlink:to="loc_us-gaap_GrossProfitAbstract_f3f1a6f1-1f13-465e-9bc8-3cb83154c314" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GrossProfit_e42049ae-7c06-4e09-afec-1f007be12f62" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GrossProfit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GrossProfitAbstract_f3f1a6f1-1f13-465e-9bc8-3cb83154c314" xlink:to="loc_us-gaap_GrossProfit_e42049ae-7c06-4e09-afec-1f007be12f62" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpensesAbstract_29bcc153-66bf-4c76-94e7-9406bea114d8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingExpensesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_ce4cd364-8145-41d0-811b-4e7df982313b" xlink:to="loc_us-gaap_OperatingExpensesAbstract_29bcc153-66bf-4c76-94e7-9406bea114d8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_5785bb8f-0966-4f3c-8286-7a9da883ded2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ResearchAndDevelopmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_29bcc153-66bf-4c76-94e7-9406bea114d8" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_5785bb8f-0966-4f3c-8286-7a9da883ded2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingAndMarketingExpense_af451f67-cb3c-4d7e-8b68-11c8fcc9e273" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SellingAndMarketingExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_29bcc153-66bf-4c76-94e7-9406bea114d8" xlink:to="loc_us-gaap_SellingAndMarketingExpense_af451f67-cb3c-4d7e-8b68-11c8fcc9e273" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpense_bbc5d81a-1ee8-4093-90f0-5b6d29c155b8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_29bcc153-66bf-4c76-94e7-9406bea114d8" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpense_bbc5d81a-1ee8-4093-90f0-5b6d29c155b8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_GainLossFromMutualCovenantNotToSueAgreementRightToUseExistingIntellectualProperty_540275ec-0b03-4148-95a8-132ed6b31ce5" xlink:href="ftnt-20211231.xsd#ftnt_GainLossFromMutualCovenantNotToSueAgreementRightToUseExistingIntellectualProperty"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_29bcc153-66bf-4c76-94e7-9406bea114d8" xlink:to="loc_ftnt_GainLossFromMutualCovenantNotToSueAgreementRightToUseExistingIntellectualProperty_540275ec-0b03-4148-95a8-132ed6b31ce5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpenses_c5220413-4965-458d-b3ca-fe14c7410421" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_29bcc153-66bf-4c76-94e7-9406bea114d8" xlink:to="loc_us-gaap_OperatingExpenses_c5220413-4965-458d-b3ca-fe14c7410421" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_a6880e6c-d4fc-43fe-ac23-50bf2ed539bd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_ce4cd364-8145-41d0-811b-4e7df982313b" xlink:to="loc_us-gaap_OperatingIncomeLoss_a6880e6c-d4fc-43fe-ac23-50bf2ed539bd" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeNet_b717aebe-67b9-4892-a118-309ab58b0c9b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InvestmentIncomeNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_ce4cd364-8145-41d0-811b-4e7df982313b" xlink:to="loc_us-gaap_InvestmentIncomeNet_b717aebe-67b9-4892-a118-309ab58b0c9b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpense_005dc24c-dfd8-4994-990c-fd5c7696a11e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InterestExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_ce4cd364-8145-41d0-811b-4e7df982313b" xlink:to="loc_us-gaap_InterestExpense_005dc24c-dfd8-4994-990c-fd5c7696a11e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_4eeff5f0-30b9-49d8-a6ba-cba94cba6192" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_ce4cd364-8145-41d0-811b-4e7df982313b" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_4eeff5f0-30b9-49d8-a6ba-cba94cba6192" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments_9fb37747-d0b8-4768-8257-dd8892897090" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_ce4cd364-8145-41d0-811b-4e7df982313b" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments_9fb37747-d0b8-4768-8257-dd8892897090" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_28e17ce3-9489-479d-b37b-2b7714bebbff" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_ce4cd364-8145-41d0-811b-4e7df982313b" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_28e17ce3-9489-479d-b37b-2b7714bebbff" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestments_d59cf6ed-a783-470f-a8e2-8345983c4b78" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeLossFromEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_ce4cd364-8145-41d0-811b-4e7df982313b" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestments_d59cf6ed-a783-470f-a8e2-8345983c4b78" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_1ad2be32-0f59-49d8-a3c5-2eadf921ff0e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_ce4cd364-8145-41d0-811b-4e7df982313b" xlink:to="loc_us-gaap_ProfitLoss_1ad2be32-0f59-49d8-a3c5-2eadf921ff0e" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAttributableToNoncontrollingInterest_651945f3-5c3b-4994-ad4d-e72276691f0d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetIncomeLossAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_ce4cd364-8145-41d0-811b-4e7df982313b" xlink:to="loc_us-gaap_NetIncomeLossAttributableToNoncontrollingInterest_651945f3-5c3b-4994-ad4d-e72276691f0d" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_2070db10-d546-45e0-902f-08fac864001f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_ce4cd364-8145-41d0-811b-4e7df982313b" xlink:to="loc_us-gaap_NetIncomeLoss_2070db10-d546-45e0-902f-08fac864001f" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareAbstract_0bacbdee-4f37-4b3c-b6f8-004511acb033" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EarningsPerShareAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_ce4cd364-8145-41d0-811b-4e7df982313b" xlink:to="loc_us-gaap_EarningsPerShareAbstract_0bacbdee-4f37-4b3c-b6f8-004511acb033" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_3a1f7eb1-3f9a-4c32-88a9-4fcc9bcf5777" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EarningsPerShareBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareAbstract_0bacbdee-4f37-4b3c-b6f8-004511acb033" xlink:to="loc_us-gaap_EarningsPerShareBasic_3a1f7eb1-3f9a-4c32-88a9-4fcc9bcf5777" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_e5348f62-008a-46ed-89ab-24a31be38d85" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareAbstract_0bacbdee-4f37-4b3c-b6f8-004511acb033" xlink:to="loc_us-gaap_EarningsPerShareDiluted_e5348f62-008a-46ed-89ab-24a31be38d85" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_e0c22f8b-ea8c-4c7d-8051-f78cbe9ce322" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_ce4cd364-8145-41d0-811b-4e7df982313b" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_e0c22f8b-ea8c-4c7d-8051-f78cbe9ce322" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_b3d4bba1-846c-459d-889c-366e66fa9241" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_e0c22f8b-ea8c-4c7d-8051-f78cbe9ce322" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_b3d4bba1-846c-459d-889c-366e66fa9241" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_949ecad5-722e-4777-948f-3f01843e0fed" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_e0c22f8b-ea8c-4c7d-8051-f78cbe9ce322" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_949ecad5-722e-4777-948f-3f01843e0fed" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_2bd5a3fd-0c9e-4996-84ec-17dc9f5acdfe" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_ce4cd364-8145-41d0-811b-4e7df982313b" xlink:to="loc_us-gaap_StatementTable_2bd5a3fd-0c9e-4996-84ec-17dc9f5acdfe" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_da84ad6d-9882-40bc-a70f-32c8f0bae7c5" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_2bd5a3fd-0c9e-4996-84ec-17dc9f5acdfe" xlink:to="loc_srt_ProductOrServiceAxis_da84ad6d-9882-40bc-a70f-32c8f0bae7c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_da84ad6d-9882-40bc-a70f-32c8f0bae7c5_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_da84ad6d-9882-40bc-a70f-32c8f0bae7c5" xlink:to="loc_srt_ProductsAndServicesDomain_da84ad6d-9882-40bc-a70f-32c8f0bae7c5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_33c3ca5c-59c6-4b1f-915d-fa8d5820ce7f" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_da84ad6d-9882-40bc-a70f-32c8f0bae7c5" xlink:to="loc_srt_ProductsAndServicesDomain_33c3ca5c-59c6-4b1f-915d-fa8d5820ce7f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductMember_bcd53ffb-f34b-4d59-902d-bb57b29ca93c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProductMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_33c3ca5c-59c6-4b1f-915d-fa8d5820ce7f" xlink:to="loc_us-gaap_ProductMember_bcd53ffb-f34b-4d59-902d-bb57b29ca93c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServiceMember_23938f90-fbef-43c6-b2b1-fcbad93cd4d8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ServiceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_33c3ca5c-59c6-4b1f-915d-fa8d5820ce7f" xlink:to="loc_us-gaap_ServiceMember_23938f90-fbef-43c6-b2b1-fcbad93cd4d8" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/ConsolidatedStatementsofStockholdersEquity" xlink:type="simple" xlink:href="ftnt-20211231.xsd#ConsolidatedStatementsofStockholdersEquity"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/ConsolidatedStatementsofStockholdersEquity" xlink:type="extended" id="i48e574a3fbc14ddc8e937f0e3c516599_ConsolidatedStatementsofStockholdersEquity">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_73a650c2-79ee-4c41-a2bc-d8913c6d73eb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_5236d38c-d0f6-4297-8b87-938de127c728" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_73a650c2-79ee-4c41-a2bc-d8913c6d73eb" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_5236d38c-d0f6-4297-8b87-938de127c728" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesIssued_e4c6232d-d984-4b35-833c-8014d25f0444" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_5236d38c-d0f6-4297-8b87-938de127c728" xlink:to="loc_us-gaap_SharesIssued_e4c6232d-d984-4b35-833c-8014d25f0444" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_f352305d-ce12-46e7-9f07-362740443439" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_5236d38c-d0f6-4297-8b87-938de127c728" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_f352305d-ce12-46e7-9f07-362740443439" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_StockIssuedDuringPeriodSharesEquityIncentivePlans_7dfa53bf-e129-4637-803d-55610de06bd4" xlink:href="ftnt-20211231.xsd#ftnt_StockIssuedDuringPeriodSharesEquityIncentivePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_5236d38c-d0f6-4297-8b87-938de127c728" xlink:to="loc_ftnt_StockIssuedDuringPeriodSharesEquityIncentivePlans_7dfa53bf-e129-4637-803d-55610de06bd4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_StockIssuedDuringPeriodValueEquityIncentivePlansNetofTaxesWithheld_d1d85726-4a62-4e42-a311-7b6c460fea5a" xlink:href="ftnt-20211231.xsd#ftnt_StockIssuedDuringPeriodValueEquityIncentivePlansNetofTaxesWithheld"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_5236d38c-d0f6-4297-8b87-938de127c728" xlink:to="loc_ftnt_StockIssuedDuringPeriodValueEquityIncentivePlansNetofTaxesWithheld_d1d85726-4a62-4e42-a311-7b6c460fea5a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_46a7ef9a-c5d9-42ab-92fa-e8f16989ab11" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_5236d38c-d0f6-4297-8b87-938de127c728" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_46a7ef9a-c5d9-42ab-92fa-e8f16989ab11" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_9beaede3-83aa-4db7-8231-8822c4913164" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_5236d38c-d0f6-4297-8b87-938de127c728" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_9beaede3-83aa-4db7-8231-8822c4913164" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_e31dff02-22f7-4351-bf09-15c66532f467" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_5236d38c-d0f6-4297-8b87-938de127c728" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_e31dff02-22f7-4351-bf09-15c66532f467" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MinorityInterestPeriodIncreaseDecrease_f9cc24ad-2ef7-42ee-a52a-74ea2a89ac1e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_MinorityInterestPeriodIncreaseDecrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_5236d38c-d0f6-4297-8b87-938de127c728" xlink:to="loc_us-gaap_MinorityInterestPeriodIncreaseDecrease_f9cc24ad-2ef7-42ee-a52a-74ea2a89ac1e" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_5e751637-bb8a-4ebf-8b94-4592b45901d0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_5236d38c-d0f6-4297-8b87-938de127c728" xlink:to="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_5e751637-bb8a-4ebf-8b94-4592b45901d0" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_d199d5e5-ed99-4bf0-9688-9808340e91b9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_5236d38c-d0f6-4297-8b87-938de127c728" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_d199d5e5-ed99-4bf0-9688-9808340e91b9" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_719b9bb9-ac6d-4441-9c20-5637697ba8a1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_5236d38c-d0f6-4297-8b87-938de127c728" xlink:to="loc_us-gaap_ProfitLoss_719b9bb9-ac6d-4441-9c20-5637697ba8a1" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesIssued_efbdac77-96a2-49ae-b6aa-0cfbe2124dbd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharesIssued"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_6fc939e5-1794-4a2d-abbd-521b84a3acd6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_3081a266-1851-489b-b2f9-ee12d88ab91d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_73a650c2-79ee-4c41-a2bc-d8913c6d73eb" xlink:to="loc_us-gaap_StatementTable_3081a266-1851-489b-b2f9-ee12d88ab91d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_500c724d-20cb-48fa-aa7f-036474058e2b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_3081a266-1851-489b-b2f9-ee12d88ab91d" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_500c724d-20cb-48fa-aa7f-036474058e2b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_500c724d-20cb-48fa-aa7f-036474058e2b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_500c724d-20cb-48fa-aa7f-036474058e2b" xlink:to="loc_us-gaap_EquityComponentDomain_500c724d-20cb-48fa-aa7f-036474058e2b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_5dd9a50b-d3a7-4b21-b5c2-e667a6c88926" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_500c724d-20cb-48fa-aa7f-036474058e2b" xlink:to="loc_us-gaap_EquityComponentDomain_5dd9a50b-d3a7-4b21-b5c2-e667a6c88926" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_f6a1f172-207b-45df-9029-663750ae9946" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_5dd9a50b-d3a7-4b21-b5c2-e667a6c88926" xlink:to="loc_us-gaap_CommonStockMember_f6a1f172-207b-45df-9029-663750ae9946" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_d588b665-545a-464e-b906-831bb0888266" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_5dd9a50b-d3a7-4b21-b5c2-e667a6c88926" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_d588b665-545a-464e-b906-831bb0888266" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_758daecd-88fb-4e87-a80f-df2ac6d65b2e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_5dd9a50b-d3a7-4b21-b5c2-e667a6c88926" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_758daecd-88fb-4e87-a80f-df2ac6d65b2e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_1b7af2ed-f268-4344-9dc5-92ad88f4f6c4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_5dd9a50b-d3a7-4b21-b5c2-e667a6c88926" xlink:to="loc_us-gaap_RetainedEarningsMember_1b7af2ed-f268-4344-9dc5-92ad88f4f6c4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NoncontrollingInterestMember_451cba30-920d-4134-adb8-b7781a74b305" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NoncontrollingInterestMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_5dd9a50b-d3a7-4b21-b5c2-e667a6c88926" xlink:to="loc_us-gaap_NoncontrollingInterestMember_451cba30-920d-4134-adb8-b7781a74b305" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_131eb70f-0b66-49c4-aa1d-e4d66948a71d" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_3081a266-1851-489b-b2f9-ee12d88ab91d" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_131eb70f-0b66-49c4-aa1d-e4d66948a71d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_131eb70f-0b66-49c4-aa1d-e4d66948a71d_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_131eb70f-0b66-49c4-aa1d-e4d66948a71d" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_131eb70f-0b66-49c4-aa1d-e4d66948a71d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_c69f70a0-9912-435b-af3a-374d831833f8" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_131eb70f-0b66-49c4-aa1d-e4d66948a71d" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_c69f70a0-9912-435b-af3a-374d831833f8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_a5f69120-73ff-4e3c-aace-0bef3031ade5" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_c69f70a0-9912-435b-af3a-374d831833f8" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_a5f69120-73ff-4e3c-aace-0bef3031ade5" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/TheCompanyandSummaryofSignificantAccountingPoliciesConcentrationofCreditRiskDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#TheCompanyandSummaryofSignificantAccountingPoliciesConcentrationofCreditRiskDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/TheCompanyandSummaryofSignificantAccountingPoliciesConcentrationofCreditRiskDetails" xlink:type="extended" id="i52f57a9068534262bd211edd55771811_TheCompanyandSummaryofSignificantAccountingPoliciesConcentrationofCreditRiskDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskLineItems_e7e0265a-f33b-444f-b6af-5a49781a7ad3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConcentrationRiskLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_a8116cfb-ce2f-4040-b58c-bbed1d40c2ce" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_e7e0265a-f33b-444f-b6af-5a49781a7ad3" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_a8116cfb-ce2f-4040-b58c-bbed1d40c2ce" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTable_04b5c3c3-ecba-4d5f-a3bc-582918d9d657" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConcentrationRiskTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ConcentrationRiskLineItems_e7e0265a-f33b-444f-b6af-5a49781a7ad3" xlink:to="loc_us-gaap_ConcentrationRiskTable_04b5c3c3-ecba-4d5f-a3bc-582918d9d657" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_b99b3d79-eb4f-46f3-a3eb-3fcb56233673" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_04b5c3c3-ecba-4d5f-a3bc-582918d9d657" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_b99b3d79-eb4f-46f3-a3eb-3fcb56233673" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_b99b3d79-eb4f-46f3-a3eb-3fcb56233673_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_b99b3d79-eb4f-46f3-a3eb-3fcb56233673" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_b99b3d79-eb4f-46f3-a3eb-3fcb56233673_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_d37e85f0-6293-472c-9c3f-11eb30ee35a7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_b99b3d79-eb4f-46f3-a3eb-3fcb56233673" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_d37e85f0-6293-472c-9c3f-11eb30ee35a7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerConcentrationRiskMember_47d47e8f-935a-4bee-9ecd-348dc2239cf0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CustomerConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_d37e85f0-6293-472c-9c3f-11eb30ee35a7" xlink:to="loc_us-gaap_CustomerConcentrationRiskMember_47d47e8f-935a-4bee-9ecd-348dc2239cf0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MajorCustomersAxis_35753159-f8de-4d3a-bb88-4258929edc36" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MajorCustomersAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_04b5c3c3-ecba-4d5f-a3bc-582918d9d657" xlink:to="loc_srt_MajorCustomersAxis_35753159-f8de-4d3a-bb88-4258929edc36" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_35753159-f8de-4d3a-bb88-4258929edc36_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_35753159-f8de-4d3a-bb88-4258929edc36" xlink:to="loc_srt_NameOfMajorCustomerDomain_35753159-f8de-4d3a-bb88-4258929edc36_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_705c1f16-b8e5-4239-a5e4-f35a2d03e724" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_35753159-f8de-4d3a-bb88-4258929edc36" xlink:to="loc_srt_NameOfMajorCustomerDomain_705c1f16-b8e5-4239-a5e4-f35a2d03e724" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ExclusiveNetworksGroupMember_2eeef1e2-890c-4179-9ae1-0d54a354aba8" xlink:href="ftnt-20211231.xsd#ftnt_ExclusiveNetworksGroupMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_705c1f16-b8e5-4239-a5e4-f35a2d03e724" xlink:to="loc_ftnt_ExclusiveNetworksGroupMember_2eeef1e2-890c-4179-9ae1-0d54a354aba8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_TDSynnexMember_a16f8385-24bc-4f75-b690-b37abd079067" xlink:href="ftnt-20211231.xsd#ftnt_TDSynnexMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_705c1f16-b8e5-4239-a5e4-f35a2d03e724" xlink:to="loc_ftnt_TDSynnexMember_a16f8385-24bc-4f75-b690-b37abd079067" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_IngramMicroMember_31e97416-0dcb-4e49-bb6d-ee9c9f3ee209" xlink:href="ftnt-20211231.xsd#ftnt_IngramMicroMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_705c1f16-b8e5-4239-a5e4-f35a2d03e724" xlink:to="loc_ftnt_IngramMicroMember_31e97416-0dcb-4e49-bb6d-ee9c9f3ee209" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_501672cd-5156-4320-94b0-1b60667038c8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ConcentrationRiskTable_04b5c3c3-ecba-4d5f-a3bc-582918d9d657" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_501672cd-5156-4320-94b0-1b60667038c8" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_501672cd-5156-4320-94b0-1b60667038c8_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_501672cd-5156-4320-94b0-1b60667038c8" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_501672cd-5156-4320-94b0-1b60667038c8_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_19abf366-f468-4d31-a0fa-5ea59fed896f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_501672cd-5156-4320-94b0-1b60667038c8" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_19abf366-f468-4d31-a0fa-5ea59fed896f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableMember_9eca3360-4e8a-4cd5-b1c1-34ee2e410f3e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccountsReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_19abf366-f468-4d31-a0fa-5ea59fed896f" xlink:to="loc_us-gaap_AccountsReceivableMember_9eca3360-4e8a-4cd5-b1c1-34ee2e410f3e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_SalesRevenueMember_3a3054dd-1fab-4e85-b389-f7166f5660ec" xlink:href="ftnt-20211231.xsd#ftnt_SalesRevenueMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_19abf366-f468-4d31-a0fa-5ea59fed896f" xlink:to="loc_ftnt_SalesRevenueMember_3a3054dd-1fab-4e85-b389-f7166f5660ec" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/TheCompanyandSummaryofSignificantAccountingPoliciesPropertyandEquipmentDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#TheCompanyandSummaryofSignificantAccountingPoliciesPropertyandEquipmentDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/TheCompanyandSummaryofSignificantAccountingPoliciesPropertyandEquipmentDetails" xlink:type="extended" id="i55271dd6c9de41079a53be2355644238_TheCompanyandSummaryofSignificantAccountingPoliciesPropertyandEquipmentDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_13b8fba4-e52d-4ee4-b42c-70e2a138930f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_2737c080-aa75-471b-87ee-9c3a0f101b3c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_13b8fba4-e52d-4ee4-b42c-70e2a138930f" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_2737c080-aa75-471b-87ee-9c3a0f101b3c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_030a8717-a8c1-4445-be51-0269d39036b3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_13b8fba4-e52d-4ee4-b42c-70e2a138930f" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_030a8717-a8c1-4445-be51-0269d39036b3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_fbb0a1d5-a5bf-4dd6-ba70-0e3a4f7fb792" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_030a8717-a8c1-4445-be51-0269d39036b3" xlink:to="loc_srt_RangeAxis_fbb0a1d5-a5bf-4dd6-ba70-0e3a4f7fb792" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_fbb0a1d5-a5bf-4dd6-ba70-0e3a4f7fb792_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_fbb0a1d5-a5bf-4dd6-ba70-0e3a4f7fb792" xlink:to="loc_srt_RangeMember_fbb0a1d5-a5bf-4dd6-ba70-0e3a4f7fb792_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_8ec34855-99ff-42d3-8e6a-584fc6358597" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_fbb0a1d5-a5bf-4dd6-ba70-0e3a4f7fb792" xlink:to="loc_srt_RangeMember_8ec34855-99ff-42d3-8e6a-584fc6358597" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_e8b384e7-1d1c-413e-ac88-0468cdd4b9c7" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_8ec34855-99ff-42d3-8e6a-584fc6358597" xlink:to="loc_srt_MinimumMember_e8b384e7-1d1c-413e-ac88-0468cdd4b9c7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_2fd1e3fa-ab78-4971-a0ec-a335b720240d" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_8ec34855-99ff-42d3-8e6a-584fc6358597" xlink:to="loc_srt_MaximumMember_2fd1e3fa-ab78-4971-a0ec-a335b720240d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_466b3a27-b00c-4309-82c1-f7b5c5ecfc01" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_030a8717-a8c1-4445-be51-0269d39036b3" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_466b3a27-b00c-4309-82c1-f7b5c5ecfc01" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_466b3a27-b00c-4309-82c1-f7b5c5ecfc01_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_466b3a27-b00c-4309-82c1-f7b5c5ecfc01" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_466b3a27-b00c-4309-82c1-f7b5c5ecfc01_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_32d4880d-d172-454b-9312-23b45534fd80" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_466b3a27-b00c-4309-82c1-f7b5c5ecfc01" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_32d4880d-d172-454b-9312-23b45534fd80" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BuildingAndBuildingImprovementsMember_905e84e4-bd7a-40f1-abc7-177c5be3b609" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BuildingAndBuildingImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_32d4880d-d172-454b-9312-23b45534fd80" xlink:to="loc_us-gaap_BuildingAndBuildingImprovementsMember_905e84e4-bd7a-40f1-abc7-177c5be3b609" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComputerEquipmentMember_9e354e3c-8430-4d63-ac5d-35a4bb81050d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ComputerEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_32d4880d-d172-454b-9312-23b45534fd80" xlink:to="loc_us-gaap_ComputerEquipmentMember_9e354e3c-8430-4d63-ac5d-35a4bb81050d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_EvaluationUnitsMember_e11919bf-8c05-4871-bdcf-55c4ff228f7a" xlink:href="ftnt-20211231.xsd#ftnt_EvaluationUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_32d4880d-d172-454b-9312-23b45534fd80" xlink:to="loc_ftnt_EvaluationUnitsMember_e11919bf-8c05-4871-bdcf-55c4ff228f7a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_3bf4296d-ef14-45e5-88ce-bcb03cfcc864" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_32d4880d-d172-454b-9312-23b45534fd80" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_3bf4296d-ef14-45e5-88ce-bcb03cfcc864" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/TheCompanyandSummaryofSignificantAccountingPoliciesOtherIntangibleAssetsDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#TheCompanyandSummaryofSignificantAccountingPoliciesOtherIntangibleAssetsDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/TheCompanyandSummaryofSignificantAccountingPoliciesOtherIntangibleAssetsDetails" xlink:type="extended" id="i04fc104c1d194cde95062a718acf5039_TheCompanyandSummaryofSignificantAccountingPoliciesOtherIntangibleAssetsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_19034dd4-3347-4163-b3ab-49e975f67e58" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_c9006cb8-c556-4d66-9efd-31de8138aa2b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_19034dd4-3347-4163-b3ab-49e975f67e58" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetUsefulLife_c9006cb8-c556-4d66-9efd-31de8138aa2b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_7e5af0a3-1ada-48d4-8aa0-b553fd0784e2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_19034dd4-3347-4163-b3ab-49e975f67e58" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_7e5af0a3-1ada-48d4-8aa0-b553fd0784e2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_7f1ef2c4-0197-4bcb-9cb1-43ab2699a94a" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_7e5af0a3-1ada-48d4-8aa0-b553fd0784e2" xlink:to="loc_srt_RangeAxis_7f1ef2c4-0197-4bcb-9cb1-43ab2699a94a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_7f1ef2c4-0197-4bcb-9cb1-43ab2699a94a_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_7f1ef2c4-0197-4bcb-9cb1-43ab2699a94a" xlink:to="loc_srt_RangeMember_7f1ef2c4-0197-4bcb-9cb1-43ab2699a94a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_bf875850-2d7c-485d-ba17-ec57c22158b2" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_7f1ef2c4-0197-4bcb-9cb1-43ab2699a94a" xlink:to="loc_srt_RangeMember_bf875850-2d7c-485d-ba17-ec57c22158b2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_2161c2e0-cae2-4908-bfca-60902daa7328" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_bf875850-2d7c-485d-ba17-ec57c22158b2" xlink:to="loc_srt_MinimumMember_2161c2e0-cae2-4908-bfca-60902daa7328" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_5e4ec910-cfc0-4b04-9dbd-56e0cae0b057" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_bf875850-2d7c-485d-ba17-ec57c22158b2" xlink:to="loc_srt_MaximumMember_5e4ec910-cfc0-4b04-9dbd-56e0cae0b057" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/TheCompanyandSummaryofSignificantAccountingPoliciesDeferredContractCostsDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#TheCompanyandSummaryofSignificantAccountingPoliciesDeferredContractCostsDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/TheCompanyandSummaryofSignificantAccountingPoliciesDeferredContractCostsDetails" xlink:type="extended" id="ic34d91a707ce4267a22133e16304855d_TheCompanyandSummaryofSignificantAccountingPoliciesDeferredContractCostsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_CommissionsExpenseLineItems_756eabc0-0787-4586-9424-01d1ed3a8a29" xlink:href="ftnt-20211231.xsd#ftnt_CommissionsExpenseLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_RevenueRecognitionPeriod_ee6d10dc-9ae0-4bc1-adcd-d36734051ae8" xlink:href="ftnt-20211231.xsd#ftnt_RevenueRecognitionPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_CommissionsExpenseLineItems_756eabc0-0787-4586-9424-01d1ed3a8a29" xlink:to="loc_ftnt_RevenueRecognitionPeriod_ee6d10dc-9ae0-4bc1-adcd-d36734051ae8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostAmortization_b1f15cf2-9e8c-4bf8-bc8d-da6edce86899" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CapitalizedContractCostAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_CommissionsExpenseLineItems_756eabc0-0787-4586-9424-01d1ed3a8a29" xlink:to="loc_us-gaap_CapitalizedContractCostAmortization_b1f15cf2-9e8c-4bf8-bc8d-da6edce86899" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostImpairmentLoss_856554ff-258e-43f0-a883-029e7b5f345a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CapitalizedContractCostImpairmentLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_CommissionsExpenseLineItems_756eabc0-0787-4586-9424-01d1ed3a8a29" xlink:to="loc_us-gaap_CapitalizedContractCostImpairmentLoss_856554ff-258e-43f0-a883-029e7b5f345a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_CommissionsExpenseTable_52de10fd-43ca-4789-b488-0bf09986c86d" xlink:href="ftnt-20211231.xsd#ftnt_CommissionsExpenseTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_ftnt_CommissionsExpenseLineItems_756eabc0-0787-4586-9424-01d1ed3a8a29" xlink:to="loc_ftnt_CommissionsExpenseTable_52de10fd-43ca-4789-b488-0bf09986c86d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_fb8d0a80-00f0-4e3f-8ca2-0199061cd5c0" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ftnt_CommissionsExpenseTable_52de10fd-43ca-4789-b488-0bf09986c86d" xlink:to="loc_srt_RangeAxis_fb8d0a80-00f0-4e3f-8ca2-0199061cd5c0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_fb8d0a80-00f0-4e3f-8ca2-0199061cd5c0_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_fb8d0a80-00f0-4e3f-8ca2-0199061cd5c0" xlink:to="loc_srt_RangeMember_fb8d0a80-00f0-4e3f-8ca2-0199061cd5c0_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_2df81398-b7d6-41fd-89a7-3d2da67e8c30" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_fb8d0a80-00f0-4e3f-8ca2-0199061cd5c0" xlink:to="loc_srt_RangeMember_2df81398-b7d6-41fd-89a7-3d2da67e8c30" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_d5388967-de3c-4346-b09a-9edfc975e80c" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_2df81398-b7d6-41fd-89a7-3d2da67e8c30" xlink:to="loc_srt_MaximumMember_d5388967-de3c-4346-b09a-9edfc975e80c" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/TheCompanyandSummaryofSignificantAccountingPoliciesRevenueRecognitionDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#TheCompanyandSummaryofSignificantAccountingPoliciesRevenueRecognitionDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/TheCompanyandSummaryofSignificantAccountingPoliciesRevenueRecognitionDetails" xlink:type="extended" id="i126178288e26461fba931b2cc279e281_TheCompanyandSummaryofSignificantAccountingPoliciesRevenueRecognitionDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_5926c82c-25a4-446b-8ceb-4ee16560d15a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_RevenueRecognitionPeriod_0207cc8c-4160-4e5d-a029-62e38243186d" xlink:href="ftnt-20211231.xsd#ftnt_RevenueRecognitionPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_5926c82c-25a4-446b-8ceb-4ee16560d15a" xlink:to="loc_ftnt_RevenueRecognitionPeriod_0207cc8c-4160-4e5d-a029-62e38243186d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerRefundLiabilityCurrent_3b2ab914-2361-4382-959d-c711d84f855b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ContractWithCustomerRefundLiabilityCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_5926c82c-25a4-446b-8ceb-4ee16560d15a" xlink:to="loc_us-gaap_ContractWithCustomerRefundLiabilityCurrent_3b2ab914-2361-4382-959d-c711d84f855b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_RevenuefromContractwithCustomersGeneralInvoicePayablePeriod_f600b41f-7fc6-4f63-9964-9299e9e6f7b9" xlink:href="ftnt-20211231.xsd#ftnt_RevenuefromContractwithCustomersGeneralInvoicePayablePeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_5926c82c-25a4-446b-8ceb-4ee16560d15a" xlink:to="loc_ftnt_RevenuefromContractwithCustomersGeneralInvoicePayablePeriod_f600b41f-7fc6-4f63-9964-9299e9e6f7b9" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_4a1b451d-a434-4b4e-895a-0d254d591316" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EntityWideInformationRevenueFromExternalCustomerLineItems_5926c82c-25a4-446b-8ceb-4ee16560d15a" xlink:to="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_4a1b451d-a434-4b4e-895a-0d254d591316" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_1f89f0c8-c890-49a8-bea8-d96b5288c6b9" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_4a1b451d-a434-4b4e-895a-0d254d591316" xlink:to="loc_srt_ProductOrServiceAxis_1f89f0c8-c890-49a8-bea8-d96b5288c6b9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_1f89f0c8-c890-49a8-bea8-d96b5288c6b9_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_1f89f0c8-c890-49a8-bea8-d96b5288c6b9" xlink:to="loc_srt_ProductsAndServicesDomain_1f89f0c8-c890-49a8-bea8-d96b5288c6b9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_f8bae023-e0f6-4e01-a37c-a1f18ec6b9df" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_1f89f0c8-c890-49a8-bea8-d96b5288c6b9" xlink:to="loc_srt_ProductsAndServicesDomain_f8bae023-e0f6-4e01-a37c-a1f18ec6b9df" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_SalesCommissionsMember_515d73b7-23f1-4e65-912f-e10b1481adc9" xlink:href="ftnt-20211231.xsd#ftnt_SalesCommissionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_f8bae023-e0f6-4e01-a37c-a1f18ec6b9df" xlink:to="loc_ftnt_SalesCommissionsMember_515d73b7-23f1-4e65-912f-e10b1481adc9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_a4797f68-d7c6-4bd6-9e03-15a1d829931e" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideInformationRevenueFromExternalCustomersByProductsAndServicesTable_4a1b451d-a434-4b4e-895a-0d254d591316" xlink:to="loc_srt_RangeAxis_a4797f68-d7c6-4bd6-9e03-15a1d829931e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_a4797f68-d7c6-4bd6-9e03-15a1d829931e_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_a4797f68-d7c6-4bd6-9e03-15a1d829931e" xlink:to="loc_srt_RangeMember_a4797f68-d7c6-4bd6-9e03-15a1d829931e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_afa050f6-31ca-445f-bb82-c576b61cddad" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_a4797f68-d7c6-4bd6-9e03-15a1d829931e" xlink:to="loc_srt_RangeMember_afa050f6-31ca-445f-bb82-c576b61cddad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_e49bd8c0-94d1-4838-ae87-4a87a7259184" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_afa050f6-31ca-445f-bb82-c576b61cddad" xlink:to="loc_srt_MinimumMember_e49bd8c0-94d1-4838-ae87-4a87a7259184" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_cf531e04-b6eb-42cd-8076-5f542b9490ef" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_afa050f6-31ca-445f-bb82-c576b61cddad" xlink:to="loc_srt_MaximumMember_cf531e04-b6eb-42cd-8076-5f542b9490ef" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/TheCompanyandSummaryofSignificantAccountingPoliciesWarrantiesDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#TheCompanyandSummaryofSignificantAccountingPoliciesWarrantiesDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/TheCompanyandSummaryofSignificantAccountingPoliciesWarrantiesDetails" xlink:type="extended" id="i66a5949c970b44069549e31e38f36cdc_TheCompanyandSummaryofSignificantAccountingPoliciesWarrantiesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_WarrantiesLineItems_f3a1fea1-83d2-42d3-9caf-89535e8e5379" xlink:href="ftnt-20211231.xsd#ftnt_WarrantiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_WarrantyLength_3db887eb-63d5-4170-9cb9-d2c7d8015d6b" xlink:href="ftnt-20211231.xsd#ftnt_WarrantyLength"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_WarrantiesLineItems_f3a1fea1-83d2-42d3-9caf-89535e8e5379" xlink:to="loc_ftnt_WarrantyLength_3db887eb-63d5-4170-9cb9-d2c7d8015d6b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ScheduleOfWarrantiesTable_99513c7e-7bbb-4a79-840a-4bbe9a815b02" xlink:href="ftnt-20211231.xsd#ftnt_ScheduleOfWarrantiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_ftnt_WarrantiesLineItems_f3a1fea1-83d2-42d3-9caf-89535e8e5379" xlink:to="loc_ftnt_ScheduleOfWarrantiesTable_99513c7e-7bbb-4a79-840a-4bbe9a815b02" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_29c454a7-4604-47ad-8695-37f3ed396d31" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ftnt_ScheduleOfWarrantiesTable_99513c7e-7bbb-4a79-840a-4bbe9a815b02" xlink:to="loc_srt_ProductOrServiceAxis_29c454a7-4604-47ad-8695-37f3ed396d31" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_29c454a7-4604-47ad-8695-37f3ed396d31_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_29c454a7-4604-47ad-8695-37f3ed396d31" xlink:to="loc_srt_ProductsAndServicesDomain_29c454a7-4604-47ad-8695-37f3ed396d31_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_afdb8f37-849d-419c-9f76-f15ced2cb338" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_29c454a7-4604-47ad-8695-37f3ed396d31" xlink:to="loc_srt_ProductsAndServicesDomain_afdb8f37-849d-419c-9f76-f15ced2cb338" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_HardwareProductWarrantyMember_f6f50b11-1579-47a5-b60c-b6e969aeba58" xlink:href="ftnt-20211231.xsd#ftnt_HardwareProductWarrantyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_afdb8f37-849d-419c-9f76-f15ced2cb338" xlink:to="loc_ftnt_HardwareProductWarrantyMember_f6f50b11-1579-47a5-b60c-b6e969aeba58" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_SoftwareProductWarrantyMember_26a88f53-9953-4e25-a757-4b8c093ac926" xlink:href="ftnt-20211231.xsd#ftnt_SoftwareProductWarrantyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_afdb8f37-849d-419c-9f76-f15ced2cb338" xlink:to="loc_ftnt_SoftwareProductWarrantyMember_26a88f53-9953-4e25-a757-4b8c093ac926" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/TheCompanyandSummaryofSignificantAccountingPoliciesRecentlyAdoptedAccountingStandardsDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#TheCompanyandSummaryofSignificantAccountingPoliciesRecentlyAdoptedAccountingStandardsDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/TheCompanyandSummaryofSignificantAccountingPoliciesRecentlyAdoptedAccountingStandardsDetails" xlink:type="extended" id="if0a1d5bc9bc1496cbcb7b266aa6f882e_TheCompanyandSummaryofSignificantAccountingPoliciesRecentlyAdoptedAccountingStandardsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillLineItems_9c962e75-fcb9-41f0-9d35-c815ebed0d82" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GoodwillLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_d024306f-9776-4af0-bb71-a8333f33502f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_9c962e75-fcb9-41f0-9d35-c815ebed0d82" xlink:to="loc_us-gaap_Goodwill_d024306f-9776-4af0-bb71-a8333f33502f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfGoodwillTable_b1120f54-73a6-48b9-8b34-2c907f01b89a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfGoodwillTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_GoodwillLineItems_9c962e75-fcb9-41f0-9d35-c815ebed0d82" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_b1120f54-73a6-48b9-8b34-2c907f01b89a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_496778f8-c70a-4a95-a617-298fec101b22" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_b1120f54-73a6-48b9-8b34-2c907f01b89a" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_496778f8-c70a-4a95-a617-298fec101b22" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_496778f8-c70a-4a95-a617-298fec101b22_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_496778f8-c70a-4a95-a617-298fec101b22" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_496778f8-c70a-4a95-a617-298fec101b22_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_8b51e26b-9e9b-4751-819a-76d262529bac" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_496778f8-c70a-4a95-a617-298fec101b22" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_8b51e26b-9e9b-4751-819a-76d262529bac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_91251750-fe02-4d73-8ca6-e46900f90957" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_8b51e26b-9e9b-4751-819a-76d262529bac" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_91251750-fe02-4d73-8ca6-e46900f90957" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_8fd96dff-5cb6-4e5a-9848-504109320210" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_b1120f54-73a6-48b9-8b34-2c907f01b89a" xlink:to="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_8fd96dff-5cb6-4e5a-9848-504109320210" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfAdoptionMember_8fd96dff-5cb6-4e5a-9848-504109320210_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TypeOfAdoptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_8fd96dff-5cb6-4e5a-9848-504109320210" xlink:to="loc_us-gaap_TypeOfAdoptionMember_8fd96dff-5cb6-4e5a-9848-504109320210_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfAdoptionMember_15d44c43-2751-4c90-b09a-e3318fff275a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TypeOfAdoptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_8fd96dff-5cb6-4e5a-9848-504109320210" xlink:to="loc_us-gaap_TypeOfAdoptionMember_15d44c43-2751-4c90-b09a-e3318fff275a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_AccountingStandardsUpdate202108Member_5510d3e3-b736-4f26-b11e-6ce914237129" xlink:href="ftnt-20211231.xsd#ftnt_AccountingStandardsUpdate202108Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfAdoptionMember_15d44c43-2751-4c90-b09a-e3318fff275a" xlink:to="loc_ftnt_AccountingStandardsUpdate202108Member_5510d3e3-b736-4f26-b11e-6ce914237129" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/RevenueRecognitionDisaggregatedRevenuesDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#RevenueRecognitionDisaggregatedRevenuesDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/RevenueRecognitionDisaggregatedRevenuesDetails" xlink:type="extended" id="i86c0f2f53ac04ae3bbe78e476269767d_RevenueRecognitionDisaggregatedRevenuesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_03ac8cec-2bc6-4f54-a2c1-4d72921074e4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_45da61b1-c062-4241-b5d1-0f1c2e244ce5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_03ac8cec-2bc6-4f54-a2c1-4d72921074e4" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_45da61b1-c062-4241-b5d1-0f1c2e244ce5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_2e139041-2088-43e5-b4f6-febeb1e9cc6c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_03ac8cec-2bc6-4f54-a2c1-4d72921074e4" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_2e139041-2088-43e5-b4f6-febeb1e9cc6c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_3d4e9154-0a25-41e1-bb14-72c248d56f06" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_2e139041-2088-43e5-b4f6-febeb1e9cc6c" xlink:to="loc_srt_ProductOrServiceAxis_3d4e9154-0a25-41e1-bb14-72c248d56f06" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_3d4e9154-0a25-41e1-bb14-72c248d56f06_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_3d4e9154-0a25-41e1-bb14-72c248d56f06" xlink:to="loc_srt_ProductsAndServicesDomain_3d4e9154-0a25-41e1-bb14-72c248d56f06_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_3b1e4244-1a47-4e7c-b808-9dbde448961c" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_3d4e9154-0a25-41e1-bb14-72c248d56f06" xlink:to="loc_srt_ProductsAndServicesDomain_3b1e4244-1a47-4e7c-b808-9dbde448961c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductMember_9c206ce9-98d5-472d-aa5e-21e3b0e34227" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProductMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_3b1e4244-1a47-4e7c-b808-9dbde448961c" xlink:to="loc_us-gaap_ProductMember_9c206ce9-98d5-472d-aa5e-21e3b0e34227" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServiceMember_8b369f99-71f1-4a8a-897f-e2a9e9e3ea76" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ServiceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_3b1e4244-1a47-4e7c-b808-9dbde448961c" xlink:to="loc_us-gaap_ServiceMember_8b369f99-71f1-4a8a-897f-e2a9e9e3ea76" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_SecuritySubscriptionMember_3359a8a9-196b-47d9-b4ec-ce99005ab51d" xlink:href="ftnt-20211231.xsd#ftnt_SecuritySubscriptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ServiceMember_8b369f99-71f1-4a8a-897f-e2a9e9e3ea76" xlink:to="loc_ftnt_SecuritySubscriptionMember_3359a8a9-196b-47d9-b4ec-ce99005ab51d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_TechnicalSupportAndOtherMember_2b592b23-a23d-4bb3-aac2-eb8785f98f7f" xlink:href="ftnt-20211231.xsd#ftnt_TechnicalSupportAndOtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ServiceMember_8b369f99-71f1-4a8a-897f-e2a9e9e3ea76" xlink:to="loc_ftnt_TechnicalSupportAndOtherMember_2b592b23-a23d-4bb3-aac2-eb8785f98f7f" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/RevenueRecognitionPerformanceObligationSatisfactionPeriodDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#RevenueRecognitionPerformanceObligationSatisfactionPeriodDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/RevenueRecognitionPerformanceObligationSatisfactionPeriodDetails" xlink:type="extended" id="i5ad8094028d44ea1b222798b28de9d46_RevenueRecognitionPerformanceObligationSatisfactionPeriodDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_27c0d605-31f8-4e10-ac27-fd6a18d55f3b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_5cb76a74-3bb7-44e9-8e1b-d6be4ededbcc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_27c0d605-31f8-4e10-ac27-fd6a18d55f3b" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_5cb76a74-3bb7-44e9-8e1b-d6be4ededbcc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_c2370ab6-0ae7-430a-989a-c2c8b5b6acf0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_27c0d605-31f8-4e10-ac27-fd6a18d55f3b" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_c2370ab6-0ae7-430a-989a-c2c8b5b6acf0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_833b5c43-0523-4f22-9d99-19fed2961bed" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_27c0d605-31f8-4e10-ac27-fd6a18d55f3b" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_833b5c43-0523-4f22-9d99-19fed2961bed" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_4b10c88f-0d0f-4948-b954-9c90a0d925b4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_833b5c43-0523-4f22-9d99-19fed2961bed" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_4b10c88f-0d0f-4948-b954-9c90a0d925b4" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/RevenueRecognitionPerformanceObligationSatisfactionPeriodDetails_1" xlink:type="simple" xlink:href="ftnt-20211231.xsd#RevenueRecognitionPerformanceObligationSatisfactionPeriodDetails_1"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/RevenueRecognitionPerformanceObligationSatisfactionPeriodDetails_1" xlink:type="extended" id="ieba285e67bb44e818ecec890faf16bcf_RevenueRecognitionPerformanceObligationSatisfactionPeriodDetails_1">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_27c0d605-31f8-4e10-ac27-fd6a18d55f3b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_5cb76a74-3bb7-44e9-8e1b-d6be4ededbcc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_27c0d605-31f8-4e10-ac27-fd6a18d55f3b" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_5cb76a74-3bb7-44e9-8e1b-d6be4ededbcc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_c2370ab6-0ae7-430a-989a-c2c8b5b6acf0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_27c0d605-31f8-4e10-ac27-fd6a18d55f3b" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_c2370ab6-0ae7-430a-989a-c2c8b5b6acf0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_833b5c43-0523-4f22-9d99-19fed2961bed" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_27c0d605-31f8-4e10-ac27-fd6a18d55f3b" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_833b5c43-0523-4f22-9d99-19fed2961bed" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/FinancialInstrumentsandFairValueInvestmentsDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#FinancialInstrumentsandFairValueInvestmentsDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/FinancialInstrumentsandFairValueInvestmentsDetails" xlink:type="extended" id="ifeacedb5c08e4505874ba4ad5e2f986b_FinancialInstrumentsandFairValueInvestmentsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_4d38545a-1ab6-4479-a40e-0cb5205431c6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_6a354679-0298-4607-a32f-0680c71ae5ee" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_4d38545a-1ab6-4479-a40e-0cb5205431c6" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_6a354679-0298-4607-a32f-0680c71ae5ee" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_d4c92153-9d55-4d5a-b8c5-42b3327c1d1d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_4d38545a-1ab6-4479-a40e-0cb5205431c6" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_d4c92153-9d55-4d5a-b8c5-42b3327c1d1d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_e2861682-e8b7-4159-ad44-883824bd05dc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_4d38545a-1ab6-4479-a40e-0cb5205431c6" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_e2861682-e8b7-4159-ad44-883824bd05dc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_dad8f082-6708-4594-910c-ca7b50e190ad" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_4d38545a-1ab6-4479-a40e-0cb5205431c6" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_dad8f082-6708-4594-910c-ca7b50e190ad" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_0f251437-0176-4847-a2a8-3c96eb274cf8" xlink:href="ftnt-20211231.xsd#ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_4d38545a-1ab6-4479-a40e-0cb5205431c6" xlink:to="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_0f251437-0176-4847-a2a8-3c96eb274cf8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months_ab800f16-b989-4562-854a-dac68522f1fa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_0f251437-0176-4847-a2a8-3c96eb274cf8" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months_ab800f16-b989-4562-854a-dac68522f1fa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_942efa18-4301-4002-b109-39ccdee25368" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_0f251437-0176-4847-a2a8-3c96eb274cf8" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_942efa18-4301-4002-b109-39ccdee25368" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger_f4a23646-a15a-4e98-b617-a107b059bd44" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_0f251437-0176-4847-a2a8-3c96eb274cf8" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger_f4a23646-a15a-4e98-b617-a107b059bd44" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_0803b9dc-ec44-415f-b3a7-a4df79d022d6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_0f251437-0176-4847-a2a8-3c96eb274cf8" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_0803b9dc-ec44-415f-b3a7-a4df79d022d6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition_7a2f7bf2-6ea9-4d32-93ee-9804b3925ec4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_0f251437-0176-4847-a2a8-3c96eb274cf8" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition_7a2f7bf2-6ea9-4d32-93ee-9804b3925ec4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_d2929f87-b9a8-4c66-b98a-8c0793a4aac3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_0f251437-0176-4847-a2a8-3c96eb274cf8" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_d2929f87-b9a8-4c66-b98a-8c0793a4aac3" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_cc962c51-b0ce-44e5-9b2a-c0d205ddaeb8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_4d38545a-1ab6-4479-a40e-0cb5205431c6" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_cc962c51-b0ce-44e5-9b2a-c0d205ddaeb8" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue_d3c916a0-fc77-4fa5-a116-551316180ade" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_cc962c51-b0ce-44e5-9b2a-c0d205ddaeb8" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue_d3c916a0-fc77-4fa5-a116-551316180ade" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_AvailableForSaleSecuritiesDebtMaturitiesAfterOneYearFairValue_f310a950-5974-4013-a051-9092b228c0a8" xlink:href="ftnt-20211231.xsd#ftnt_AvailableForSaleSecuritiesDebtMaturitiesAfterOneYearFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_cc962c51-b0ce-44e5-9b2a-c0d205ddaeb8" xlink:to="loc_ftnt_AvailableForSaleSecuritiesDebtMaturitiesAfterOneYearFairValue_f310a950-5974-4013-a051-9092b228c0a8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_6141f400-842e-48d6-a46c-868a9a42f487" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesTable_35a70fbc-9483-4347-9583-dc1fbc21277b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_4d38545a-1ab6-4479-a40e-0cb5205431c6" xlink:to="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesTable_35a70fbc-9483-4347-9583-dc1fbc21277b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_89367d2b-8fa1-45eb-bacc-c87f5e8b63e6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesTable_35a70fbc-9483-4347-9583-dc1fbc21277b" xlink:to="loc_us-gaap_FinancialInstrumentAxis_89367d2b-8fa1-45eb-bacc-c87f5e8b63e6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_89367d2b-8fa1-45eb-bacc-c87f5e8b63e6_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_89367d2b-8fa1-45eb-bacc-c87f5e8b63e6" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_89367d2b-8fa1-45eb-bacc-c87f5e8b63e6_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_3c9d9343-bb69-462c-9cf7-e5affabb348b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_89367d2b-8fa1-45eb-bacc-c87f5e8b63e6" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_3c9d9343-bb69-462c-9cf7-e5affabb348b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_aa8332e7-3872-4330-8d66-ee29cf1c847e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_3c9d9343-bb69-462c-9cf7-e5affabb348b" xlink:to="loc_us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_aa8332e7-3872-4330-8d66-ee29cf1c847e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_47a4d903-3613-44fe-b3b5-75bc9aeeffa4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_3c9d9343-bb69-462c-9cf7-e5affabb348b" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_47a4d903-3613-44fe-b3b5-75bc9aeeffa4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_a0da7e6b-ed71-4c59-b22b-85c915f3ccff" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_USTreasuryAndGovernmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_3c9d9343-bb69-462c-9cf7-e5affabb348b" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_a0da7e6b-ed71-4c59-b22b-85c915f3ccff" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CertificatesOfDepositMember_047b54b1-222b-4bba-a87c-713a1c8dc15b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CertificatesOfDepositMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_3c9d9343-bb69-462c-9cf7-e5affabb348b" xlink:to="loc_us-gaap_CertificatesOfDepositMember_047b54b1-222b-4bba-a87c-713a1c8dc15b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MunicipalBondsMember_5d88a20f-5004-4176-9e65-f7690e76fcb9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_MunicipalBondsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_3c9d9343-bb69-462c-9cf7-e5affabb348b" xlink:to="loc_us-gaap_MunicipalBondsMember_5d88a20f-5004-4176-9e65-f7690e76fcb9" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/FinancialInstrumentsandFairValueFairValueMeasurementsDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#FinancialInstrumentsandFairValueFairValueMeasurementsDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/FinancialInstrumentsandFairValueFairValueMeasurementsDetails" xlink:type="extended" id="i4d0d723c1cb246aab93701ee66e3c6b5_FinancialInstrumentsandFairValueFairValueMeasurementsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_863264eb-66c8-4e00-8950-7d81df3f4301" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_fc67c75a-8090-4a50-bdf2-37c368d8aaa7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_863264eb-66c8-4e00-8950-7d81df3f4301" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_fc67c75a-8090-4a50-bdf2-37c368d8aaa7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_1eb3ce3a-a8c9-416a-8ea5-8343268a8a8c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_863264eb-66c8-4e00-8950-7d81df3f4301" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_1eb3ce3a-a8c9-416a-8ea5-8343268a8a8c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_076b8bc0-6397-4fcb-a69f-eca2410f1263" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_863264eb-66c8-4e00-8950-7d81df3f4301" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_076b8bc0-6397-4fcb-a69f-eca2410f1263" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementBasisAxis_432bb6b0-7435-4568-ac30-6ff0c17f98e9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueByMeasurementBasisAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_076b8bc0-6397-4fcb-a69f-eca2410f1263" xlink:to="loc_us-gaap_FairValueByMeasurementBasisAxis_432bb6b0-7435-4568-ac30-6ff0c17f98e9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_432bb6b0-7435-4568-ac30-6ff0c17f98e9_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_432bb6b0-7435-4568-ac30-6ff0c17f98e9" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_432bb6b0-7435-4568-ac30-6ff0c17f98e9_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureItemAmountsDomain_df618a24-84e6-4443-a63e-156c5f666e5e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueDisclosureItemAmountsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_432bb6b0-7435-4568-ac30-6ff0c17f98e9" xlink:to="loc_us-gaap_FairValueDisclosureItemAmountsDomain_df618a24-84e6-4443-a63e-156c5f666e5e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_2e7f3b3e-abf8-4e8c-8086-5a87a146ba4c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_df618a24-84e6-4443-a63e-156c5f666e5e" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_2e7f3b3e-abf8-4e8c-8086-5a87a146ba4c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_10a9764c-59de-4b0d-a44a-9989327e7da8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_df618a24-84e6-4443-a63e-156c5f666e5e" xlink:to="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_10a9764c-59de-4b0d-a44a-9989327e7da8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_1553ae51-0609-4769-8f6b-29d5909a3f2f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CarryingReportedAmountFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_df618a24-84e6-4443-a63e-156c5f666e5e" xlink:to="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_1553ae51-0609-4769-8f6b-29d5909a3f2f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_c2d4b8ff-aabe-4df0-b8c3-64f659dae31c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_076b8bc0-6397-4fcb-a69f-eca2410f1263" xlink:to="loc_us-gaap_FinancialInstrumentAxis_c2d4b8ff-aabe-4df0-b8c3-64f659dae31c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_c2d4b8ff-aabe-4df0-b8c3-64f659dae31c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_c2d4b8ff-aabe-4df0-b8c3-64f659dae31c" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_c2d4b8ff-aabe-4df0-b8c3-64f659dae31c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_9b6fdbd5-b9cb-4b26-97f3-af589ddbcc32" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_c2d4b8ff-aabe-4df0-b8c3-64f659dae31c" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_9b6fdbd5-b9cb-4b26-97f3-af589ddbcc32" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_d0494bf0-06b0-413f-bfcb-139f3e75b73c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_9b6fdbd5-b9cb-4b26-97f3-af589ddbcc32" xlink:to="loc_us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_d0494bf0-06b0-413f-bfcb-139f3e75b73c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_b6cd7d39-391b-4916-a3de-6ba1268feb23" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_9b6fdbd5-b9cb-4b26-97f3-af589ddbcc32" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_b6cd7d39-391b-4916-a3de-6ba1268feb23" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_2ca043b5-4832-4f70-b32e-712d5ed5209f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_USTreasuryAndGovernmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_9b6fdbd5-b9cb-4b26-97f3-af589ddbcc32" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_2ca043b5-4832-4f70-b32e-712d5ed5209f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CertificatesOfDepositMember_0878e1ff-3df4-43cb-abf6-1b2943e26873" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CertificatesOfDepositMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_9b6fdbd5-b9cb-4b26-97f3-af589ddbcc32" xlink:to="loc_us-gaap_CertificatesOfDepositMember_0878e1ff-3df4-43cb-abf6-1b2943e26873" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_MoneyMarketFundsExcludingCommercialPaperMember_52a1069f-09f5-4e34-aa30-bf41c36e424b" xlink:href="ftnt-20211231.xsd#ftnt_MoneyMarketFundsExcludingCommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_9b6fdbd5-b9cb-4b26-97f3-af589ddbcc32" xlink:to="loc_ftnt_MoneyMarketFundsExcludingCommercialPaperMember_52a1069f-09f5-4e34-aa30-bf41c36e424b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_MunicipalBondsMember_0c1193e2-e9b9-4edc-99e8-38126865844b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_MunicipalBondsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_9b6fdbd5-b9cb-4b26-97f3-af589ddbcc32" xlink:to="loc_us-gaap_MunicipalBondsMember_0c1193e2-e9b9-4edc-99e8-38126865844b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesMember_7364ab2a-2ed3-40d4-a706-0fccc90ba08f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquitySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_9b6fdbd5-b9cb-4b26-97f3-af589ddbcc32" xlink:to="loc_us-gaap_EquitySecuritiesMember_7364ab2a-2ed3-40d4-a706-0fccc90ba08f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_57bbfaa2-b980-4922-bf36-09cfa69cee3c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_076b8bc0-6397-4fcb-a69f-eca2410f1263" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_57bbfaa2-b980-4922-bf36-09cfa69cee3c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_57bbfaa2-b980-4922-bf36-09cfa69cee3c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_57bbfaa2-b980-4922-bf36-09cfa69cee3c" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_57bbfaa2-b980-4922-bf36-09cfa69cee3c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_651344df-2442-4ffb-9948-5b4acdff047d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_57bbfaa2-b980-4922-bf36-09cfa69cee3c" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_651344df-2442-4ffb-9948-5b4acdff047d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_5ecc4536-9fc6-4f15-b988-53db68ce1e12" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_651344df-2442-4ffb-9948-5b4acdff047d" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_5ecc4536-9fc6-4f15-b988-53db68ce1e12" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_e0681852-0696-4463-a274-17447d99800f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_076b8bc0-6397-4fcb-a69f-eca2410f1263" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_e0681852-0696-4463-a274-17447d99800f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_e0681852-0696-4463-a274-17447d99800f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_e0681852-0696-4463-a274-17447d99800f" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_e0681852-0696-4463-a274-17447d99800f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_df6508f5-5f10-432d-8b7f-10b8d8b7bf15" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_e0681852-0696-4463-a274-17447d99800f" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_df6508f5-5f10-432d-8b7f-10b8d8b7bf15" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_CashEquivalentsLineItemMember_0e8eeb03-5d1a-4d74-9df0-0a2744dfedec" xlink:href="ftnt-20211231.xsd#ftnt_CashEquivalentsLineItemMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_df6508f5-5f10-432d-8b7f-10b8d8b7bf15" xlink:to="loc_ftnt_CashEquivalentsLineItemMember_0e8eeb03-5d1a-4d74-9df0-0a2744dfedec" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_MarketableEquitySecuritiesLineItemMember_f059431d-2d60-471a-a699-55248a6e3174" xlink:href="ftnt-20211231.xsd#ftnt_MarketableEquitySecuritiesLineItemMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_df6508f5-5f10-432d-8b7f-10b8d8b7bf15" xlink:to="loc_ftnt_MarketableEquitySecuritiesLineItemMember_f059431d-2d60-471a-a699-55248a6e3174" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ShortTermInvestmentsLineItemMember_fd83edde-a115-4369-85ea-ed16c26c4e2f" xlink:href="ftnt-20211231.xsd#ftnt_ShortTermInvestmentsLineItemMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_df6508f5-5f10-432d-8b7f-10b8d8b7bf15" xlink:to="loc_ftnt_ShortTermInvestmentsLineItemMember_fd83edde-a115-4369-85ea-ed16c26c4e2f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_LongTermInvestmentsLineItemMember_406e16a2-094f-4962-b328-a41e29ce3b90" xlink:href="ftnt-20211231.xsd#ftnt_LongTermInvestmentsLineItemMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_df6508f5-5f10-432d-8b7f-10b8d8b7bf15" xlink:to="loc_ftnt_LongTermInvestmentsLineItemMember_406e16a2-094f-4962-b328-a41e29ce3b90" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_69bf16c8-a5d6-499f-a79f-366ff901ad49" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_076b8bc0-6397-4fcb-a69f-eca2410f1263" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_69bf16c8-a5d6-499f-a79f-366ff901ad49" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_69bf16c8-a5d6-499f-a79f-366ff901ad49_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_69bf16c8-a5d6-499f-a79f-366ff901ad49" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_69bf16c8-a5d6-499f-a79f-366ff901ad49_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_d576f73b-fa90-46bd-9155-d8f10a92b992" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_69bf16c8-a5d6-499f-a79f-366ff901ad49" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_d576f73b-fa90-46bd-9155-d8f10a92b992" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_d7765bb4-12dd-4666-87ff-e3b5aa1a162a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_d576f73b-fa90-46bd-9155-d8f10a92b992" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_d7765bb4-12dd-4666-87ff-e3b5aa1a162a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_c97e43de-872f-4602-a04a-2e3853a74520" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_d576f73b-fa90-46bd-9155-d8f10a92b992" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_c97e43de-872f-4602-a04a-2e3853a74520" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_d7d29e81-9b88-4e1d-8139-3de2a4e9be1c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_d576f73b-fa90-46bd-9155-d8f10a92b992" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_d7d29e81-9b88-4e1d-8139-3de2a4e9be1c" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/PropertyandEquipmentNetScheduleofPropertyPlantandEquipmentNetDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#PropertyandEquipmentNetScheduleofPropertyPlantandEquipmentNetDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/PropertyandEquipmentNetScheduleofPropertyPlantandEquipmentNetDetails" xlink:type="extended" id="i510d9cb5f16d4abdbbb80d1fa57fa356_PropertyandEquipmentNetScheduleofPropertyPlantandEquipmentNetDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_518b9994-9c39-408d-8cd5-32563d5c634b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract_80db9415-52d7-4ced-8854-930c35fbaa28" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_518b9994-9c39-408d-8cd5-32563d5c634b" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract_80db9415-52d7-4ced-8854-930c35fbaa28" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_b2d1c478-3291-4379-9ab5-bee8e551aa82" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract_80db9415-52d7-4ced-8854-930c35fbaa28" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_b2d1c478-3291-4379-9ab5-bee8e551aa82" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_d0674067-fb94-4f99-9471-db3458fefb9d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract_80db9415-52d7-4ced-8854-930c35fbaa28" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_d0674067-fb94-4f99-9471-db3458fefb9d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_014e3e18-f0ea-49b7-b69c-b0df9356013a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract_80db9415-52d7-4ced-8854-930c35fbaa28" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_014e3e18-f0ea-49b7-b69c-b0df9356013a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_4038d3e9-2a1e-4019-8929-861897c9d5cd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_518b9994-9c39-408d-8cd5-32563d5c634b" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_4038d3e9-2a1e-4019-8929-861897c9d5cd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_d5ae568d-8da6-442b-91fe-0475925227ea" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_4038d3e9-2a1e-4019-8929-861897c9d5cd" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_d5ae568d-8da6-442b-91fe-0475925227ea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_d5ae568d-8da6-442b-91fe-0475925227ea_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_d5ae568d-8da6-442b-91fe-0475925227ea" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_d5ae568d-8da6-442b-91fe-0475925227ea_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_48c2ee1f-d7cc-4f40-8d9d-37e110f2a43b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_d5ae568d-8da6-442b-91fe-0475925227ea" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_48c2ee1f-d7cc-4f40-8d9d-37e110f2a43b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LandMember_78e259e8-4b44-49c5-9bb1-68070553c431" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LandMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_48c2ee1f-d7cc-4f40-8d9d-37e110f2a43b" xlink:to="loc_us-gaap_LandMember_78e259e8-4b44-49c5-9bb1-68070553c431" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BuildingAndBuildingImprovementsMember_ab632d71-50a6-4f08-9a4c-df3292406dba" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BuildingAndBuildingImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_48c2ee1f-d7cc-4f40-8d9d-37e110f2a43b" xlink:to="loc_us-gaap_BuildingAndBuildingImprovementsMember_ab632d71-50a6-4f08-9a4c-df3292406dba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComputerEquipmentMember_663e619b-cb66-4ab8-937c-db49edd53afe" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ComputerEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_48c2ee1f-d7cc-4f40-8d9d-37e110f2a43b" xlink:to="loc_us-gaap_ComputerEquipmentMember_663e619b-cb66-4ab8-937c-db49edd53afe" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdsAndLeaseholdImprovementsMember_1b2d03ba-efe3-4327-bfaa-26db613c2146" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LeaseholdsAndLeaseholdImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_48c2ee1f-d7cc-4f40-8d9d-37e110f2a43b" xlink:to="loc_us-gaap_LeaseholdsAndLeaseholdImprovementsMember_1b2d03ba-efe3-4327-bfaa-26db613c2146" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_EvaluationUnitsMember_97121504-4414-45c2-b5c0-9ecc46d81b7b" xlink:href="ftnt-20211231.xsd#ftnt_EvaluationUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_48c2ee1f-d7cc-4f40-8d9d-37e110f2a43b" xlink:to="loc_ftnt_EvaluationUnitsMember_97121504-4414-45c2-b5c0-9ecc46d81b7b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_0aa4e2db-112e-45d6-b417-bdfe9433e217" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_48c2ee1f-d7cc-4f40-8d9d-37e110f2a43b" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_0aa4e2db-112e-45d6-b417-bdfe9433e217" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConstructionInProgressMember_8ad7aaff-c2cd-44df-a7e1-0148a668654d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConstructionInProgressMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_48c2ee1f-d7cc-4f40-8d9d-37e110f2a43b" xlink:to="loc_us-gaap_ConstructionInProgressMember_8ad7aaff-c2cd-44df-a7e1-0148a668654d" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/PropertyandEquipmentNetAdditionalInformationDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#PropertyandEquipmentNetAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/PropertyandEquipmentNetAdditionalInformationDetails" xlink:type="extended" id="ie982c769a009460d8586fc981d97fab1_PropertyandEquipmentNetAdditionalInformationDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_0c443219-3099-4363-a987-443109b4e405" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_828e01ef-a81e-4de5-af1e-eeb4a5607392" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_0c443219-3099-4363-a987-443109b4e405" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentUsefulLife_828e01ef-a81e-4de5-af1e-eeb4a5607392" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_1ba16de1-d07d-4692-9444-68e0c47d98bd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_0c443219-3099-4363-a987-443109b4e405" xlink:to="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_1ba16de1-d07d-4692-9444-68e0c47d98bd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_3a8175c8-6978-440b-8082-17bc0c86a169" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_0c443219-3099-4363-a987-443109b4e405" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_3a8175c8-6978-440b-8082-17bc0c86a169" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Depreciation_e00a6738-aeab-4600-b14f-7745605e7f15" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Depreciation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_0c443219-3099-4363-a987-443109b4e405" xlink:to="loc_us-gaap_Depreciation_e00a6738-aeab-4600-b14f-7745605e7f15" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_00b62abc-5871-444b-a5c5-ba6f156cb2c3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_0c443219-3099-4363-a987-443109b4e405" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_00b62abc-5871-444b-a5c5-ba6f156cb2c3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_e2565671-9f15-480e-bd4e-f444fed26fbb" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_00b62abc-5871-444b-a5c5-ba6f156cb2c3" xlink:to="loc_srt_RangeAxis_e2565671-9f15-480e-bd4e-f444fed26fbb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_e2565671-9f15-480e-bd4e-f444fed26fbb_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_e2565671-9f15-480e-bd4e-f444fed26fbb" xlink:to="loc_srt_RangeMember_e2565671-9f15-480e-bd4e-f444fed26fbb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_2b3d8e55-5e71-4b31-8dfd-94e89f182795" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_e2565671-9f15-480e-bd4e-f444fed26fbb" xlink:to="loc_srt_RangeMember_2b3d8e55-5e71-4b31-8dfd-94e89f182795" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_a72255cd-2840-4cba-a0c0-564070a626e2" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_2b3d8e55-5e71-4b31-8dfd-94e89f182795" xlink:to="loc_srt_MinimumMember_a72255cd-2840-4cba-a0c0-564070a626e2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_7e25edec-6c75-4a62-8f5e-9e74cfa8d86e" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_2b3d8e55-5e71-4b31-8dfd-94e89f182795" xlink:to="loc_srt_MaximumMember_7e25edec-6c75-4a62-8f5e-9e74cfa8d86e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_2265aab0-ff4e-4665-a22f-345a34fa6cca" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_00b62abc-5871-444b-a5c5-ba6f156cb2c3" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_2265aab0-ff4e-4665-a22f-345a34fa6cca" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_2265aab0-ff4e-4665-a22f-345a34fa6cca_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_2265aab0-ff4e-4665-a22f-345a34fa6cca" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_2265aab0-ff4e-4665-a22f-345a34fa6cca_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_961c0623-2fba-4b34-9c01-9d9f9e095a94" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_2265aab0-ff4e-4665-a22f-345a34fa6cca" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_961c0623-2fba-4b34-9c01-9d9f9e095a94" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BuildingAndBuildingImprovementsMember_1642930f-9d01-401d-a86d-dd2a2e15e6e7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BuildingAndBuildingImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_961c0623-2fba-4b34-9c01-9d9f9e095a94" xlink:to="loc_us-gaap_BuildingAndBuildingImprovementsMember_1642930f-9d01-401d-a86d-dd2a2e15e6e7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_083f168c-569e-49dd-81cf-a688da66a27d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_961c0623-2fba-4b34-9c01-9d9f9e095a94" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_083f168c-569e-49dd-81cf-a688da66a27d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LandMember_43631f94-9fa6-4329-b921-995f32f42edc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LandMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_961c0623-2fba-4b34-9c01-9d9f9e095a94" xlink:to="loc_us-gaap_LandMember_43631f94-9fa6-4329-b921-995f32f42edc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConstructionInProgressMember_d4a3dda7-c56e-4f5e-a4ec-688f746d7038" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ConstructionInProgressMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_961c0623-2fba-4b34-9c01-9d9f9e095a94" xlink:to="loc_us-gaap_ConstructionInProgressMember_d4a3dda7-c56e-4f5e-a4ec-688f746d7038" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_2773677f-2ec5-41eb-a740-47c647e5ce3a" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_00b62abc-5871-444b-a5c5-ba6f156cb2c3" xlink:to="loc_srt_StatementGeographicalAxis_2773677f-2ec5-41eb-a740-47c647e5ce3a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_2773677f-2ec5-41eb-a740-47c647e5ce3a_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_2773677f-2ec5-41eb-a740-47c647e5ce3a" xlink:to="loc_srt_SegmentGeographicalDomain_2773677f-2ec5-41eb-a740-47c647e5ce3a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_ea317be0-9356-4ac4-bb40-5920265454f8" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_2773677f-2ec5-41eb-a740-47c647e5ce3a" xlink:to="loc_srt_SegmentGeographicalDomain_ea317be0-9356-4ac4-bb40-5920265454f8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_USCanadaAndFranceMember_ac349148-d866-47c1-877c-57681a9173ca" xlink:href="ftnt-20211231.xsd#ftnt_USCanadaAndFranceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_ea317be0-9356-4ac4-bb40-5920265454f8" xlink:to="loc_ftnt_USCanadaAndFranceMember_ac349148-d866-47c1-877c-57681a9173ca" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/InvestmentsinPrivatelyHeldCompaniesDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#InvestmentsinPrivatelyHeldCompaniesDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/InvestmentsinPrivatelyHeldCompaniesDetails" xlink:type="extended" id="i3028f20548ea42cc9b46109791ff8062_InvestmentsinPrivatelyHeldCompaniesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_1197bdcf-4c80-4a12-aa25-3cb47723fd44" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireEquityMethodInvestments_7e44161a-ab7c-449c-9d06-851cb1f31d99" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsToAcquireEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_1197bdcf-4c80-4a12-aa25-3cb47723fd44" xlink:to="loc_us-gaap_PaymentsToAcquireEquityMethodInvestments_7e44161a-ab7c-449c-9d06-851cb1f31d99" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_4db95283-d87e-403c-9fc5-6ceb64576065" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_1197bdcf-4c80-4a12-aa25-3cb47723fd44" xlink:to="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_4db95283-d87e-403c-9fc5-6ceb64576065" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestments_8f9863ba-0b8d-43e8-84a5-48280187b46e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeLossFromEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_1197bdcf-4c80-4a12-aa25-3cb47723fd44" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestments_8f9863ba-0b8d-43e8-84a5-48280187b46e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount_0a84efcb-2ea5-441a-86dd-3b178ae8e337" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_1197bdcf-4c80-4a12-aa25-3cb47723fd44" xlink:to="loc_us-gaap_EquitySecuritiesWithoutReadilyDeterminableFairValueAmount_0a84efcb-2ea5-441a-86dd-3b178ae8e337" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_004940bf-1347-454c-abf5-729c151a8060" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfEquityMethodInvestmentsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsLineItems_1197bdcf-4c80-4a12-aa25-3cb47723fd44" xlink:to="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_004940bf-1347-454c-abf5-729c151a8060" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_2e9af5eb-8b5f-47e9-818c-28c2bd2e3324" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEquityMethodInvestmentsTable_004940bf-1347-454c-abf5-729c151a8060" xlink:to="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_2e9af5eb-8b5f-47e9-818c-28c2bd2e3324" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_2e9af5eb-8b5f-47e9-818c-28c2bd2e3324_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_2e9af5eb-8b5f-47e9-818c-28c2bd2e3324" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_2e9af5eb-8b5f-47e9-818c-28c2bd2e3324_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_c98c250b-3b60-43ac-a406-f56ef19e2dce" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_2e9af5eb-8b5f-47e9-818c-28c2bd2e3324" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_c98c250b-3b60-43ac-a406-f56ef19e2dce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_LinksysMember_918354ce-786f-4e7c-a23d-cd10f67a9867" xlink:href="ftnt-20211231.xsd#ftnt_LinksysMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_c98c250b-3b60-43ac-a406-f56ef19e2dce" xlink:to="loc_ftnt_LinksysMember_918354ce-786f-4e7c-a23d-cd10f67a9867" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/BusinessCombinationsAdditionalInformationDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#BusinessCombinationsAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/BusinessCombinationsAdditionalInformationDetails" xlink:type="extended" id="if0cd8149519b406db15775747e388664_BusinessCombinationsAdditionalInformationDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_d2723cb0-44aa-448f-b90c-0d98439e936b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired_2db4f320-98f3-4e4e-8144-790b38db7728" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d2723cb0-44aa-448f-b90c-0d98439e936b" xlink:to="loc_us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired_2db4f320-98f3-4e4e-8144-790b38db7728" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_bb06567b-8ebe-409e-98b4-2456ce4ba835" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d2723cb0-44aa-448f-b90c-0d98439e936b" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_bb06567b-8ebe-409e-98b4-2456ce4ba835" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationAcquisitionOfLessThan100PercentNoncontrollingInterestFairValue_a0838681-24ac-4706-9737-eca3b600df99" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationAcquisitionOfLessThan100PercentNoncontrollingInterestFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d2723cb0-44aa-448f-b90c-0d98439e936b" xlink:to="loc_us-gaap_BusinessCombinationAcquisitionOfLessThan100PercentNoncontrollingInterestFairValue_a0838681-24ac-4706-9737-eca3b600df99" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities_fd27a3db-9ee6-404b-9ff5-8e2cf15d9d67" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d2723cb0-44aa-448f-b90c-0d98439e936b" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilities_fd27a3db-9ee6-404b-9ff5-8e2cf15d9d67" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_08b0f243-54fc-4945-827a-94f7284815cc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d2723cb0-44aa-448f-b90c-0d98439e936b" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_08b0f243-54fc-4945-827a-94f7284815cc" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_f235c183-6630-4c72-9ff6-94da7e14b374" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d2723cb0-44aa-448f-b90c-0d98439e936b" xlink:to="loc_us-gaap_Goodwill_f235c183-6630-4c72-9ff6-94da7e14b374" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_7fdeb74e-935e-4872-92bd-1125639989f9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d2723cb0-44aa-448f-b90c-0d98439e936b" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_7fdeb74e-935e-4872-92bd-1125639989f9" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherAssetsLiabilitiesNet_d38ff5bd-2cc2-40e2-a773-ace0fd1d186a" xlink:href="ftnt-20211231.xsd#ftnt_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherAssetsLiabilitiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d2723cb0-44aa-448f-b90c-0d98439e936b" xlink:to="loc_ftnt_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherAssetsLiabilitiesNet_d38ff5bd-2cc2-40e2-a773-ace0fd1d186a" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_21ee217e-325d-4993-a1ed-1a7ce85eb0ac" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d2723cb0-44aa-448f-b90c-0d98439e936b" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_21ee217e-325d-4993-a1ed-1a7ce85eb0ac" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_6a973cb8-7fdd-49c7-89d0-c07e39c7c941" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d2723cb0-44aa-448f-b90c-0d98439e936b" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_6a973cb8-7fdd-49c7-89d0-c07e39c7c941" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_6f7d97e4-6647-4d79-bd0e-e6a13705972a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d2723cb0-44aa-448f-b90c-0d98439e936b" xlink:to="loc_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_6f7d97e4-6647-4d79-bd0e-e6a13705972a" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_BusinessCombinationConsiderationTransferredAmountHeldinIndemnityEscrowPortionExpectedtobeHeldforTwelveMonthsAfterClosingDate_1e62b25f-f6b9-474c-888a-154f6151a94b" xlink:href="ftnt-20211231.xsd#ftnt_BusinessCombinationConsiderationTransferredAmountHeldinIndemnityEscrowPortionExpectedtobeHeldforTwelveMonthsAfterClosingDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d2723cb0-44aa-448f-b90c-0d98439e936b" xlink:to="loc_ftnt_BusinessCombinationConsiderationTransferredAmountHeldinIndemnityEscrowPortionExpectedtobeHeldforTwelveMonthsAfterClosingDate_1e62b25f-f6b9-474c-888a-154f6151a94b" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_BusinessCombinationConsiderationTransferredAmountHeldInIndemnityEscrowPortionReceivedByTheCompany_ea52a412-053a-449f-8753-8e88dddff5ef" xlink:href="ftnt-20211231.xsd#ftnt_BusinessCombinationConsiderationTransferredAmountHeldInIndemnityEscrowPortionReceivedByTheCompany"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d2723cb0-44aa-448f-b90c-0d98439e936b" xlink:to="loc_ftnt_BusinessCombinationConsiderationTransferredAmountHeldInIndemnityEscrowPortionReceivedByTheCompany_ea52a412-053a-449f-8753-8e88dddff5ef" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_BusinessCombinationLiabilitiesIncurred_abef0637-2933-4ab2-991f-fe715e19978c" xlink:href="ftnt-20211231.xsd#ftnt_BusinessCombinationLiabilitiesIncurred"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d2723cb0-44aa-448f-b90c-0d98439e936b" xlink:to="loc_ftnt_BusinessCombinationLiabilitiesIncurred_abef0637-2933-4ab2-991f-fe715e19978c" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred_285e2d6b-a0de-4772-911f-40a20a3e6bc4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d2723cb0-44aa-448f-b90c-0d98439e936b" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferredLiabilitiesIncurred_285e2d6b-a0de-4772-911f-40a20a3e6bc4" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_BusinessCombinationCompensationExpenseLiabilitiesIncurred_72006e14-b78f-4b7f-aa71-b5846d50fc80" xlink:href="ftnt-20211231.xsd#ftnt_BusinessCombinationCompensationExpenseLiabilitiesIncurred"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d2723cb0-44aa-448f-b90c-0d98439e936b" xlink:to="loc_ftnt_BusinessCombinationCompensationExpenseLiabilitiesIncurred_72006e14-b78f-4b7f-aa71-b5846d50fc80" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_BusinessCombinationConsiderationTransferredAmountHeldinIndemnityEscrow_9b4a1fb0-3fee-4b90-9d71-ad7076acb0d4" xlink:href="ftnt-20211231.xsd#ftnt_BusinessCombinationConsiderationTransferredAmountHeldinIndemnityEscrow"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d2723cb0-44aa-448f-b90c-0d98439e936b" xlink:to="loc_ftnt_BusinessCombinationConsiderationTransferredAmountHeldinIndemnityEscrow_9b4a1fb0-3fee-4b90-9d71-ad7076acb0d4" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_BusinessCombinationConsiderationTransferredAmountHeldinIndemnityEscrowPortionExpectedtobeHeldforThirtySixMonthsAfterClosingDate_a92ba820-8db8-4bba-b1e7-4f21554d77da" xlink:href="ftnt-20211231.xsd#ftnt_BusinessCombinationConsiderationTransferredAmountHeldinIndemnityEscrowPortionExpectedtobeHeldforThirtySixMonthsAfterClosingDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d2723cb0-44aa-448f-b90c-0d98439e936b" xlink:to="loc_ftnt_BusinessCombinationConsiderationTransferredAmountHeldinIndemnityEscrowPortionExpectedtobeHeldforThirtySixMonthsAfterClosingDate_a92ba820-8db8-4bba-b1e7-4f21554d77da" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ProceedsFromReleaseOfIndemnityEscrowRelatedToBusinessCombination_e3bcdb36-1b84-4589-8c8a-7f72afe54012" xlink:href="ftnt-20211231.xsd#ftnt_ProceedsFromReleaseOfIndemnityEscrowRelatedToBusinessCombination"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d2723cb0-44aa-448f-b90c-0d98439e936b" xlink:to="loc_ftnt_ProceedsFromReleaseOfIndemnityEscrowRelatedToBusinessCombination_e3bcdb36-1b84-4589-8c8a-7f72afe54012" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_d88974b4-295c-41df-86d4-d7907c63bbaf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_d2723cb0-44aa-448f-b90c-0d98439e936b" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_d88974b4-295c-41df-86d4-d7907c63bbaf" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_bec6d644-ee7e-4882-af27-a12ea84bb1a3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_d88974b4-295c-41df-86d4-d7907c63bbaf" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_bec6d644-ee7e-4882-af27-a12ea84bb1a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_bec6d644-ee7e-4882-af27-a12ea84bb1a3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_bec6d644-ee7e-4882-af27-a12ea84bb1a3" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_bec6d644-ee7e-4882-af27-a12ea84bb1a3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_531b7395-43dd-47e3-970b-7bdc346f6df0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_bec6d644-ee7e-4882-af27-a12ea84bb1a3" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_531b7395-43dd-47e3-970b-7bdc346f6df0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_AlaxaIANetworksCorporationMember_97123c5a-017e-46c1-b34a-a9388893f19d" xlink:href="ftnt-20211231.xsd#ftnt_AlaxaIANetworksCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_531b7395-43dd-47e3-970b-7bdc346f6df0" xlink:to="loc_ftnt_AlaxaIANetworksCorporationMember_97123c5a-017e-46c1-b34a-a9388893f19d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ShieldXNetworksIncShieldXMember_1ceeee25-d762-4502-8d2c-307bcf17c03c" xlink:href="ftnt-20211231.xsd#ftnt_ShieldXNetworksIncShieldXMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_531b7395-43dd-47e3-970b-7bdc346f6df0" xlink:to="loc_ftnt_ShieldXNetworksIncShieldXMember_1ceeee25-d762-4502-8d2c-307bcf17c03c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_PanoptaHoldingsLLCMember_9c11326c-8f25-48de-a4fb-73715c4f7403" xlink:href="ftnt-20211231.xsd#ftnt_PanoptaHoldingsLLCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_531b7395-43dd-47e3-970b-7bdc346f6df0" xlink:to="loc_ftnt_PanoptaHoldingsLLCMember_9c11326c-8f25-48de-a4fb-73715c4f7403" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_OPAQNetworksIncOPAQMember_2d79e2e9-d2ae-4b3e-b86e-85692a2914b4" xlink:href="ftnt-20211231.xsd#ftnt_OPAQNetworksIncOPAQMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_531b7395-43dd-47e3-970b-7bdc346f6df0" xlink:to="loc_ftnt_OPAQNetworksIncOPAQMember_2d79e2e9-d2ae-4b3e-b86e-85692a2914b4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_CyberSponseInc.Member_3124c5a4-4e35-47e5-8d74-e52d7bf69132" xlink:href="ftnt-20211231.xsd#ftnt_CyberSponseInc.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_531b7395-43dd-47e3-970b-7bdc346f6df0" xlink:to="loc_ftnt_CyberSponseInc.Member_3124c5a4-4e35-47e5-8d74-e52d7bf69132" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_EnSiloLtd.Member_37eac4b1-0634-4349-99da-81891c117db7" xlink:href="ftnt-20211231.xsd#ftnt_EnSiloLtd.Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_531b7395-43dd-47e3-970b-7bdc346f6df0" xlink:to="loc_ftnt_EnSiloLtd.Member_37eac4b1-0634-4349-99da-81891c117db7" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_4d26012b-7a04-4363-93d6-3f19ff0e669a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_d88974b4-295c-41df-86d4-d7907c63bbaf" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_4d26012b-7a04-4363-93d6-3f19ff0e669a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_4d26012b-7a04-4363-93d6-3f19ff0e669a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_4d26012b-7a04-4363-93d6-3f19ff0e669a" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_4d26012b-7a04-4363-93d6-3f19ff0e669a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_2b6ce9d1-dd3d-4b0d-aa64-9e40a6a19b05" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_4d26012b-7a04-4363-93d6-3f19ff0e669a" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_2b6ce9d1-dd3d-4b0d-aa64-9e40a6a19b05" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_fddceffc-67d1-4b16-b5a0-7d4135ff2415" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DevelopedTechnologyRightsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_2b6ce9d1-dd3d-4b0d-aa64-9e40a6a19b05" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_fddceffc-67d1-4b16-b5a0-7d4135ff2415" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/BusinessCombinationsScheduleofRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#BusinessCombinationsScheduleofRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/BusinessCombinationsScheduleofRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedDetails" xlink:type="extended" id="ifdde7802f7dc4e76a8c4d7f8df1a2cc3_BusinessCombinationsScheduleofRecognizedIdentifiedAssetsAcquiredandLiabilitiesAssumedDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_b160109c-b341-4b40-8d05-29ff86f9e559" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract_5d524075-2a8a-4c4e-931f-595d1d8f2a66" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b160109c-b341-4b40-8d05-29ff86f9e559" xlink:to="loc_us-gaap_AssetsAbstract_5d524075-2a8a-4c4e-931f-595d1d8f2a66" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_1f254b67-8272-4478-8eb5-1796a5b6f968" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_5d524075-2a8a-4c4e-931f-595d1d8f2a66" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_1f254b67-8272-4478-8eb5-1796a5b6f968" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_99affc2e-2714-4139-a04b-4fabd723b2ec" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_5d524075-2a8a-4c4e-931f-595d1d8f2a66" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_99affc2e-2714-4139-a04b-4fabd723b2ec" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInventory_2500a72d-1c26-4f9c-bec3-fd0e43be037b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInventory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_5d524075-2a8a-4c4e-931f-595d1d8f2a66" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInventory_2500a72d-1c26-4f9c-bec3-fd0e43be037b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets_d77b6656-8487-4388-b4a2-0fe6eab12096" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_5d524075-2a8a-4c4e-931f-595d1d8f2a66" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets_d77b6656-8487-4388-b4a2-0fe6eab12096" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_ce73e8a4-0047-476e-a6d3-ffef3ff2f6ba" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_5d524075-2a8a-4c4e-931f-595d1d8f2a66" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_ce73e8a4-0047-476e-a6d3-ffef3ff2f6ba" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_9437fa69-3e14-400d-a728-47f94b6efd83" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_5d524075-2a8a-4c4e-931f-595d1d8f2a66" xlink:to="loc_us-gaap_Goodwill_9437fa69-3e14-400d-a728-47f94b6efd83" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_2bae9731-f32a-4867-99ee-e22f1ab72215" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_5d524075-2a8a-4c4e-931f-595d1d8f2a66" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_2bae9731-f32a-4867-99ee-e22f1ab72215" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets_1ef77344-9aa9-4611-8298-f1be013f905c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_5d524075-2a8a-4c4e-931f-595d1d8f2a66" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherNoncurrentAssets_1ef77344-9aa9-4611-8298-f1be013f905c" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets_194b3f48-5b0b-4e9e-a857-70acbbf272ac" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_5d524075-2a8a-4c4e-931f-595d1d8f2a66" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets_194b3f48-5b0b-4e9e-a857-70acbbf272ac" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesAbstract_9e4ffa34-7724-4249-a28f-71b7403624e1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LiabilitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b160109c-b341-4b40-8d05-29ff86f9e559" xlink:to="loc_us-gaap_LiabilitiesAbstract_9e4ffa34-7724-4249-a28f-71b7403624e1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_76a128bb-3418-4319-aa9b-372356c48c23" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_9e4ffa34-7724-4249-a28f-71b7403624e1" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_76a128bb-3418-4319-aa9b-372356c48c23" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesLongTermDebt_7fdb926f-d8d0-4d06-8d3c-e48b5adc3052" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesLongTermDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_9e4ffa34-7724-4249-a28f-71b7403624e1" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesLongTermDebt_7fdb926f-d8d0-4d06-8d3c-e48b5adc3052" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_7d425cf3-a415-4618-b303-528d529c9c5f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_9e4ffa34-7724-4249-a28f-71b7403624e1" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_7d425cf3-a415-4618-b303-528d529c9c5f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther_67706a8d-b52a-492e-8d1c-150a91c718fb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_9e4ffa34-7724-4249-a28f-71b7403624e1" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNoncurrentLiabilitiesOther_67706a8d-b52a-492e-8d1c-150a91c718fb" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_b58e3458-0305-4b2a-809a-768265b381f3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAbstract_9e4ffa34-7724-4249-a28f-71b7403624e1" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_b58e3458-0305-4b2a-809a-768265b381f3" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationAcquisitionOfLessThan100PercentNoncontrollingInterestFairValue_60143d52-9950-4eb7-a187-d5e6f6bdcc0e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationAcquisitionOfLessThan100PercentNoncontrollingInterestFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b160109c-b341-4b40-8d05-29ff86f9e559" xlink:to="loc_us-gaap_BusinessCombinationAcquisitionOfLessThan100PercentNoncontrollingInterestFairValue_60143d52-9950-4eb7-a187-d5e6f6bdcc0e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_c2c3f6d4-6db8-46e9-bd2f-d3953c96c0a9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b160109c-b341-4b40-8d05-29ff86f9e559" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_c2c3f6d4-6db8-46e9-bd2f-d3953c96c0a9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_e3dc23ea-ab90-445a-996f-fa0cd2baf3cd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_b160109c-b341-4b40-8d05-29ff86f9e559" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_e3dc23ea-ab90-445a-996f-fa0cd2baf3cd" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_0f271f9b-a004-4460-84ce-b45b5b1f2c47" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_e3dc23ea-ab90-445a-996f-fa0cd2baf3cd" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_0f271f9b-a004-4460-84ce-b45b5b1f2c47" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0f271f9b-a004-4460-84ce-b45b5b1f2c47_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_0f271f9b-a004-4460-84ce-b45b5b1f2c47" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_0f271f9b-a004-4460-84ce-b45b5b1f2c47_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_ec5e23e4-68e4-41d3-a9c1-18bf76fd1df5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_0f271f9b-a004-4460-84ce-b45b5b1f2c47" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_ec5e23e4-68e4-41d3-a9c1-18bf76fd1df5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_AlaxaIANetworksCorporationMember_336b90ab-869c-46bf-b4a4-04462d16fba1" xlink:href="ftnt-20211231.xsd#ftnt_AlaxaIANetworksCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_ec5e23e4-68e4-41d3-a9c1-18bf76fd1df5" xlink:to="loc_ftnt_AlaxaIANetworksCorporationMember_336b90ab-869c-46bf-b4a4-04462d16fba1" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/BusinessCombinationsScheduleofAcquiredFiniteLivedIntangibleAssetsDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#BusinessCombinationsScheduleofAcquiredFiniteLivedIntangibleAssetsDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/BusinessCombinationsScheduleofAcquiredFiniteLivedIntangibleAssetsDetails" xlink:type="extended" id="ie3d49ad08c2642f6b999c31afbab6f62_BusinessCombinationsScheduleofAcquiredFiniteLivedIntangibleAssetsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_81d1db4f-4216-43f9-856f-07c8e319f740" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_beafae51-f86b-47f7-b9ae-a94ca5c365f9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FinitelivedIntangibleAssetsAcquired1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_81d1db4f-4216-43f9-856f-07c8e319f740" xlink:to="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_beafae51-f86b-47f7-b9ae-a94ca5c365f9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_27a53b02-9998-470a-975a-40363e1ef3d2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_81d1db4f-4216-43f9-856f-07c8e319f740" xlink:to="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_27a53b02-9998-470a-975a-40363e1ef3d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_71e73bd8-b270-4c39-bc26-f7f19f128f01" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_81d1db4f-4216-43f9-856f-07c8e319f740" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_71e73bd8-b270-4c39-bc26-f7f19f128f01" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_5ca21185-f5a8-4a82-881a-666a282097b6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_81d1db4f-4216-43f9-856f-07c8e319f740" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_5ca21185-f5a8-4a82-881a-666a282097b6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_42f14ea7-d288-40b6-9d09-7940716ded63" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_5ca21185-f5a8-4a82-881a-666a282097b6" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_42f14ea7-d288-40b6-9d09-7940716ded63" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_42f14ea7-d288-40b6-9d09-7940716ded63_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_42f14ea7-d288-40b6-9d09-7940716ded63" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_42f14ea7-d288-40b6-9d09-7940716ded63_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_cd68d077-9393-49ec-adc8-97561dbe5ac9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_42f14ea7-d288-40b6-9d09-7940716ded63" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_cd68d077-9393-49ec-adc8-97561dbe5ac9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_AlaxaIANetworksCorporationMember_cc5f05ba-e68d-4e04-a888-e67f4946c4d6" xlink:href="ftnt-20211231.xsd#ftnt_AlaxaIANetworksCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_cd68d077-9393-49ec-adc8-97561dbe5ac9" xlink:to="loc_ftnt_AlaxaIANetworksCorporationMember_cc5f05ba-e68d-4e04-a888-e67f4946c4d6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_f4c21c6f-06c2-4c48-9482-56fb6e623c86" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_5ca21185-f5a8-4a82-881a-666a282097b6" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_f4c21c6f-06c2-4c48-9482-56fb6e623c86" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_f4c21c6f-06c2-4c48-9482-56fb6e623c86_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_f4c21c6f-06c2-4c48-9482-56fb6e623c86" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_f4c21c6f-06c2-4c48-9482-56fb6e623c86_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_4f42e223-ac68-476c-afb8-d9fcaa1019f8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_f4c21c6f-06c2-4c48-9482-56fb6e623c86" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_4f42e223-ac68-476c-afb8-d9fcaa1019f8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_fd46d36e-7c1d-4df8-8921-1a7c4c50cae2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DevelopedTechnologyRightsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_4f42e223-ac68-476c-afb8-d9fcaa1019f8" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_fd46d36e-7c1d-4df8-8921-1a7c4c50cae2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_cf9a04ba-0081-4728-af13-2ef43c12166f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_4f42e223-ac68-476c-afb8-d9fcaa1019f8" xlink:to="loc_us-gaap_CustomerRelationshipsMember_cf9a04ba-0081-4728-af13-2ef43c12166f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_f9663fc5-5dcd-4810-8382-b9dabf996a41" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_4f42e223-ac68-476c-afb8-d9fcaa1019f8" xlink:to="loc_us-gaap_TradeNamesMember_f9663fc5-5dcd-4810-8382-b9dabf996a41" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_BacklogMember_7189ec7e-ae6c-4a0a-93f0-c71b16dde4da" xlink:href="ftnt-20211231.xsd#ftnt_BacklogMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_4f42e223-ac68-476c-afb8-d9fcaa1019f8" xlink:to="loc_ftnt_BacklogMember_7189ec7e-ae6c-4a0a-93f0-c71b16dde4da" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/BusinessCombinationsProFormaInformationDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#BusinessCombinationsProFormaInformationDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/BusinessCombinationsProFormaInformationDetails" xlink:type="extended" id="i0bd313f0a5814555ac9d7b1bbde21f9d_BusinessCombinationsProFormaInformationDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_09a73b0e-d5bf-4b6e-b07e-6b8d13065854" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionsProFormaRevenue_55f65e8d-880d-4339-abe1-f31d14c9c927" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionsProFormaRevenue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_09a73b0e-d5bf-4b6e-b07e-6b8d13065854" xlink:to="loc_us-gaap_BusinessAcquisitionsProFormaRevenue_55f65e8d-880d-4339-abe1-f31d14c9c927" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss_e5658d6c-db20-4e0d-b3bf-a9accc780303" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_09a73b0e-d5bf-4b6e-b07e-6b8d13065854" xlink:to="loc_us-gaap_BusinessAcquisitionsProFormaNetIncomeLoss_e5658d6c-db20-4e0d-b3bf-a9accc780303" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_ce36b21b-8841-4437-8c2f-1aede4aeccad" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_09a73b0e-d5bf-4b6e-b07e-6b8d13065854" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_ce36b21b-8841-4437-8c2f-1aede4aeccad" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_03aec593-f119-4866-86dc-cfb8b4590ac1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_ce36b21b-8841-4437-8c2f-1aede4aeccad" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_03aec593-f119-4866-86dc-cfb8b4590ac1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_03aec593-f119-4866-86dc-cfb8b4590ac1_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_03aec593-f119-4866-86dc-cfb8b4590ac1" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_03aec593-f119-4866-86dc-cfb8b4590ac1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_cc93e40d-038d-4ae4-9ceb-a6a41d4213df" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_03aec593-f119-4866-86dc-cfb8b4590ac1" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_cc93e40d-038d-4ae4-9ceb-a6a41d4213df" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_AlaxaIANetworksCorporationMember_31f59ff5-de6b-4980-a56a-7022760dc6bb" xlink:href="ftnt-20211231.xsd#ftnt_AlaxaIANetworksCorporationMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_cc93e40d-038d-4ae4-9ceb-a6a41d4213df" xlink:to="loc_ftnt_AlaxaIANetworksCorporationMember_31f59ff5-de6b-4980-a56a-7022760dc6bb" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/GoodwillandOtherIntangibleAssetsNetGoodwillDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#GoodwillandOtherIntangibleAssetsNetGoodwillDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/GoodwillandOtherIntangibleAssetsNetGoodwillDetails" xlink:type="extended" id="ide73d2b7eeb5463aa42ae6e2d02fa63e_GoodwillandOtherIntangibleAssetsNetGoodwillDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillLineItems_59383cc9-6d77-4364-947f-bb8aae837e79" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GoodwillLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillRollForward_bfee885a-6ab3-40e0-a5ba-ab85e20b31dd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GoodwillRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_59383cc9-6d77-4364-947f-bb8aae837e79" xlink:to="loc_us-gaap_GoodwillRollForward_bfee885a-6ab3-40e0-a5ba-ab85e20b31dd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_7765dcff-a1e1-4e3c-9ed7-96533b151fe8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_bfee885a-6ab3-40e0-a5ba-ab85e20b31dd" xlink:to="loc_us-gaap_Goodwill_7765dcff-a1e1-4e3c-9ed7-96533b151fe8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillAcquiredDuringPeriod_5d325862-0841-4df8-8cf3-28020068ff23" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GoodwillAcquiredDuringPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_bfee885a-6ab3-40e0-a5ba-ab85e20b31dd" xlink:to="loc_us-gaap_GoodwillAcquiredDuringPeriod_5d325862-0841-4df8-8cf3-28020068ff23" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillForeignCurrencyTranslationGainLoss_a60ef5e8-5b20-4f9d-b2a3-03f1ff175dfa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GoodwillForeignCurrencyTranslationGainLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillRollForward_bfee885a-6ab3-40e0-a5ba-ab85e20b31dd" xlink:to="loc_us-gaap_GoodwillForeignCurrencyTranslationGainLoss_a60ef5e8-5b20-4f9d-b2a3-03f1ff175dfa" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_90735e5d-9abb-4275-b3e4-80a5b75a61db" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_Goodwill"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GoodwillImpairmentLoss_3613b4b2-36c1-4e80-9964-65d0c46abd84" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GoodwillImpairmentLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GoodwillLineItems_59383cc9-6d77-4364-947f-bb8aae837e79" xlink:to="loc_us-gaap_GoodwillImpairmentLoss_3613b4b2-36c1-4e80-9964-65d0c46abd84" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfGoodwillTable_1ea0057f-2912-4625-8b32-cf5e740580ed" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfGoodwillTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_GoodwillLineItems_59383cc9-6d77-4364-947f-bb8aae837e79" xlink:to="loc_us-gaap_ScheduleOfGoodwillTable_1ea0057f-2912-4625-8b32-cf5e740580ed" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_d7a2b003-6599-4b93-b93b-916faafd8b9d" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_1ea0057f-2912-4625-8b32-cf5e740580ed" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_d7a2b003-6599-4b93-b93b-916faafd8b9d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_d7a2b003-6599-4b93-b93b-916faafd8b9d_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_d7a2b003-6599-4b93-b93b-916faafd8b9d" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_d7a2b003-6599-4b93-b93b-916faafd8b9d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_ffa26140-7950-4a35-a4a1-b76a76a26ab1" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionAxis_d7a2b003-6599-4b93-b93b-916faafd8b9d" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_ffa26140-7950-4a35-a4a1-b76a76a26ab1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_aef5f2c0-7204-4c33-b882-1879b57de7bc" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_CumulativeEffectPeriodOfAdoptionDomain_ffa26140-7950-4a35-a4a1-b76a76a26ab1" xlink:to="loc_srt_CumulativeEffectPeriodOfAdoptionAdjustmentMember_aef5f2c0-7204-4c33-b882-1879b57de7bc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_2d631831-79cf-428d-959f-7ac7b67f00af" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfGoodwillTable_1ea0057f-2912-4625-8b32-cf5e740580ed" xlink:to="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_2d631831-79cf-428d-959f-7ac7b67f00af" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfAdoptionMember_2d631831-79cf-428d-959f-7ac7b67f00af_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TypeOfAdoptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_2d631831-79cf-428d-959f-7ac7b67f00af" xlink:to="loc_us-gaap_TypeOfAdoptionMember_2d631831-79cf-428d-959f-7ac7b67f00af_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TypeOfAdoptionMember_3971e2ea-95ee-4e99-a00e-9d2c73e07c4c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TypeOfAdoptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_2d631831-79cf-428d-959f-7ac7b67f00af" xlink:to="loc_us-gaap_TypeOfAdoptionMember_3971e2ea-95ee-4e99-a00e-9d2c73e07c4c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_AccountingStandardsUpdate202108Member_9bfb32d0-8345-4386-94cc-85ac10f0abff" xlink:href="ftnt-20211231.xsd#ftnt_AccountingStandardsUpdate202108Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfAdoptionMember_3971e2ea-95ee-4e99-a00e-9d2c73e07c4c" xlink:to="loc_ftnt_AccountingStandardsUpdate202108Member_9bfb32d0-8345-4386-94cc-85ac10f0abff" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/GoodwillandOtherIntangibleAssetsNetOtherIntangibleAssetsNetDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#GoodwillandOtherIntangibleAssetsNetOtherIntangibleAssetsNetDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/GoodwillandOtherIntangibleAssetsNetOtherIntangibleAssetsNetDetails" xlink:type="extended" id="ieb5e448ab36f4cff9dd68e014aa34cc3_GoodwillandOtherIntangibleAssetsNetOtherIntangibleAssetsNetDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_65d58c84-f1ef-4c23-9944-5fc5dd154ef3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_afb41227-16a1-4410-9c80-6e499a0012b6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_65d58c84-f1ef-4c23-9944-5fc5dd154ef3" xlink:to="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_afb41227-16a1-4410-9c80-6e499a0012b6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_66d16d2b-1e9d-4447-84fe-41ecc1f01dbf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_65d58c84-f1ef-4c23-9944-5fc5dd154ef3" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_66d16d2b-1e9d-4447-84fe-41ecc1f01dbf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_9e6a2100-6dae-4905-95b2-764f650047cd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_65d58c84-f1ef-4c23-9944-5fc5dd154ef3" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_9e6a2100-6dae-4905-95b2-764f650047cd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_0e73e839-7213-4761-8527-461a89108169" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_65d58c84-f1ef-4c23-9944-5fc5dd154ef3" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_0e73e839-7213-4761-8527-461a89108169" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_4c11ad76-e173-4076-b15e-6a47df85ccd8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AmortizationOfIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_65d58c84-f1ef-4c23-9944-5fc5dd154ef3" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_4c11ad76-e173-4076-b15e-6a47df85ccd8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_04c1d7ae-3dd0-485f-b606-b83a2743cda9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_65d58c84-f1ef-4c23-9944-5fc5dd154ef3" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_04c1d7ae-3dd0-485f-b606-b83a2743cda9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_b3d838a4-2207-4bd7-9627-38d24fbf7517" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_04c1d7ae-3dd0-485f-b606-b83a2743cda9" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_b3d838a4-2207-4bd7-9627-38d24fbf7517" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_b3d838a4-2207-4bd7-9627-38d24fbf7517_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_b3d838a4-2207-4bd7-9627-38d24fbf7517" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_b3d838a4-2207-4bd7-9627-38d24fbf7517_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_2ebf4a98-ac79-43df-9e1c-86f599109091" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_b3d838a4-2207-4bd7-9627-38d24fbf7517" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_2ebf4a98-ac79-43df-9e1c-86f599109091" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_18fe725a-8cbf-423a-8759-5007477513dc" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DevelopedTechnologyRightsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_2ebf4a98-ac79-43df-9e1c-86f599109091" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_18fe725a-8cbf-423a-8759-5007477513dc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_459e9d58-efb3-4214-8d5b-964ea5b1ac60" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_2ebf4a98-ac79-43df-9e1c-86f599109091" xlink:to="loc_us-gaap_CustomerRelationshipsMember_459e9d58-efb3-4214-8d5b-964ea5b1ac60" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_26b66aaf-4c60-4858-86f5-a30f73e2f2e4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_2ebf4a98-ac79-43df-9e1c-86f599109091" xlink:to="loc_us-gaap_TradeNamesMember_26b66aaf-4c60-4858-86f5-a30f73e2f2e4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_BacklogMember_e225eff8-e5fb-4144-acbc-bcce4b20c1a4" xlink:href="ftnt-20211231.xsd#ftnt_BacklogMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_2ebf4a98-ac79-43df-9e1c-86f599109091" xlink:to="loc_ftnt_BacklogMember_e225eff8-e5fb-4144-acbc-bcce4b20c1a4" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/NetIncomePerShareCalculationofBasicandDilutedDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#NetIncomePerShareCalculationofBasicandDilutedDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/NetIncomePerShareCalculationofBasicandDilutedDetails" xlink:type="extended" id="ic634dbc82bc044898abca352fb82648a_NetIncomePerShareCalculationofBasicandDilutedDetails">
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_EarningsPerShareLineItems_d5ff206d-a043-4a2b-b691-f334f3642221" xlink:href="ftnt-20211231.xsd#ftnt_EarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProfitLoss_f13597ba-7ae3-4c29-a531-4426ebc68788" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ProfitLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_d5ff206d-a043-4a2b-b691-f334f3642221" xlink:to="loc_us-gaap_ProfitLoss_f13597ba-7ae3-4c29-a531-4426ebc68788" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLossAttributableToNoncontrollingInterest_9f06e616-a35e-4aee-91b0-b985d04c271d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetIncomeLossAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_d5ff206d-a043-4a2b-b691-f334f3642221" xlink:to="loc_us-gaap_NetIncomeLossAttributableToNoncontrollingInterest_9f06e616-a35e-4aee-91b0-b985d04c271d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_2d2c3f5a-a53f-4f8c-92e2-f64971677555" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_d5ff206d-a043-4a2b-b691-f334f3642221" xlink:to="loc_us-gaap_NetIncomeLoss_2d2c3f5a-a53f-4f8c-92e2-f64971677555" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasicAbstract_32532cbf-d853-4be7-a0a1-5da0808e0369" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasicAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_d5ff206d-a043-4a2b-b691-f334f3642221" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasicAbstract_32532cbf-d853-4be7-a0a1-5da0808e0369" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_b8ba9b52-37b8-4c14-b051-790b9f261ffa" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasicAbstract_32532cbf-d853-4be7-a0a1-5da0808e0369" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_b8ba9b52-37b8-4c14-b051-790b9f261ffa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_ab885807-9a43-4ba4-aa88-f73032da0b71" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_d5ff206d-a043-4a2b-b691-f334f3642221" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_ab885807-9a43-4ba4-aa88-f73032da0b71" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_4900800b-53f4-45e9-b686-7014cd8fb6b0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_32dec03a-012e-4d38-ba31-3ae6481f3938" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_d5ff206d-a043-4a2b-b691-f334f3642221" xlink:to="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_32dec03a-012e-4d38-ba31-3ae6481f3938" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_2a2718d7-75bd-4392-a136-d4aed6f2f03f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_32dec03a-012e-4d38-ba31-3ae6481f3938" xlink:to="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_2a2718d7-75bd-4392-a136-d4aed6f2f03f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_ec822dbe-f910-4386-bf0e-76148294e0c6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_d5ff206d-a043-4a2b-b691-f334f3642221" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_ec822dbe-f910-4386-bf0e-76148294e0c6" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasicAndDilutedAbstract_074927d1-9f63-451e-9e3d-3617535db671" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EarningsPerShareBasicAndDilutedAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_d5ff206d-a043-4a2b-b691-f334f3642221" xlink:to="loc_us-gaap_EarningsPerShareBasicAndDilutedAbstract_074927d1-9f63-451e-9e3d-3617535db671" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_30cf2b09-4ee9-47db-9aed-a6bd2d53011c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EarningsPerShareBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAndDilutedAbstract_074927d1-9f63-451e-9e3d-3617535db671" xlink:to="loc_us-gaap_EarningsPerShareBasic_30cf2b09-4ee9-47db-9aed-a6bd2d53011c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_bbdc1535-3545-4800-b9e7-411952000652" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareBasicAndDilutedAbstract_074927d1-9f63-451e-9e3d-3617535db671" xlink:to="loc_us-gaap_EarningsPerShareDiluted_bbdc1535-3545-4800-b9e7-411952000652" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ScheduleOfEarningsPerShareTable_42713335-3eac-4eb6-b15d-c89df428d0f3" xlink:href="ftnt-20211231.xsd#ftnt_ScheduleOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_ftnt_EarningsPerShareLineItems_d5ff206d-a043-4a2b-b691-f334f3642221" xlink:to="loc_ftnt_ScheduleOfEarningsPerShareTable_42713335-3eac-4eb6-b15d-c89df428d0f3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_5c9e7ab6-883b-4b07-bc24-3b918207d620" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ftnt_ScheduleOfEarningsPerShareTable_42713335-3eac-4eb6-b15d-c89df428d0f3" xlink:to="loc_us-gaap_AwardTypeAxis_5c9e7ab6-883b-4b07-bc24-3b918207d620" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_5c9e7ab6-883b-4b07-bc24-3b918207d620_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_5c9e7ab6-883b-4b07-bc24-3b918207d620" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_5c9e7ab6-883b-4b07-bc24-3b918207d620_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_75f54b73-8d4c-44e5-abe5-c2a27c128f36" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_5c9e7ab6-883b-4b07-bc24-3b918207d620" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_75f54b73-8d4c-44e5-abe5-c2a27c128f36" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_38837b5e-168d-46c3-a84c-7f06d95a3b1e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_75f54b73-8d4c-44e5-abe5-c2a27c128f36" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_38837b5e-168d-46c3-a84c-7f06d95a3b1e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_47a4c160-fc76-4266-ad89-8352904bbba8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_75f54b73-8d4c-44e5-abe5-c2a27c128f36" xlink:to="loc_us-gaap_EmployeeStockOptionMember_47a4c160-fc76-4266-ad89-8352904bbba8" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/NetIncomePerShareAntiDilutiveSecuritiesDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#NetIncomePerShareAntiDilutiveSecuritiesDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/NetIncomePerShareAntiDilutiveSecuritiesDetails" xlink:type="extended" id="i86651fe9e25e4374990d719b17e041e7_NetIncomePerShareAntiDilutiveSecuritiesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_93572bee-3512-4dcc-80ba-e19ec2d73e5a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_91b6ab9b-bfdd-40fb-a8ae-19402ed86293" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_93572bee-3512-4dcc-80ba-e19ec2d73e5a" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_91b6ab9b-bfdd-40fb-a8ae-19402ed86293" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_b454c49c-cf9f-42c6-a4c3-0a3e62030ebe" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_93572bee-3512-4dcc-80ba-e19ec2d73e5a" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_b454c49c-cf9f-42c6-a4c3-0a3e62030ebe" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_68467659-5df8-4806-9d32-603abfb6f3a4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_b454c49c-cf9f-42c6-a4c3-0a3e62030ebe" xlink:to="loc_us-gaap_AwardTypeAxis_68467659-5df8-4806-9d32-603abfb6f3a4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_68467659-5df8-4806-9d32-603abfb6f3a4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_68467659-5df8-4806-9d32-603abfb6f3a4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_68467659-5df8-4806-9d32-603abfb6f3a4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c13d9b03-c0dd-44cd-bfe3-92ddbf4616d6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_68467659-5df8-4806-9d32-603abfb6f3a4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c13d9b03-c0dd-44cd-bfe3-92ddbf4616d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_bf5fceec-7ad0-4c1d-a04e-230ae18ae8d2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c13d9b03-c0dd-44cd-bfe3-92ddbf4616d6" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_bf5fceec-7ad0-4c1d-a04e-230ae18ae8d2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_37565f25-6224-4f75-8400-51d1f97653a6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c13d9b03-c0dd-44cd-bfe3-92ddbf4616d6" xlink:to="loc_us-gaap_EmployeeStockOptionMember_37565f25-6224-4f75-8400-51d1f97653a6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_a9260881-ffa8-45cb-969f-6699c4b85a9d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_b454c49c-cf9f-42c6-a4c3-0a3e62030ebe" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_a9260881-ffa8-45cb-969f-6699c4b85a9d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_a9260881-ffa8-45cb-969f-6699c4b85a9d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_a9260881-ffa8-45cb-969f-6699c4b85a9d" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_a9260881-ffa8-45cb-969f-6699c4b85a9d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_b7184a4c-6d9f-4ac2-91e1-e0e21619d7e1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_a9260881-ffa8-45cb-969f-6699c4b85a9d" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_b7184a4c-6d9f-4ac2-91e1-e0e21619d7e1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockCompensationPlanMember_5a0ed5d8-f2b9-40b1-b394-d911b375b4a3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockCompensationPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_b7184a4c-6d9f-4ac2-91e1-e0e21619d7e1" xlink:to="loc_us-gaap_StockCompensationPlanMember_5a0ed5d8-f2b9-40b1-b394-d911b375b4a3" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/LeasesAdditionalInformationDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#LeasesAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/LeasesAdditionalInformationDetails" xlink:type="extended" id="i0eaf6b5551cd4ba8a2c209cd18ab2fb8_LeasesAdditionalInformationDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_f9a20fae-f2fe-415b-aee6-39e93ba06b05" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeLeaseDescriptionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_OperatingLeaseRemainingLeaseTerm_423fb391-f7b6-4274-ad68-2b42f8723792" xlink:href="ftnt-20211231.xsd#ftnt_OperatingLeaseRemainingLeaseTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_f9a20fae-f2fe-415b-aee6-39e93ba06b05" xlink:to="loc_ftnt_OperatingLeaseRemainingLeaseTerm_423fb391-f7b6-4274-ad68-2b42f8723792" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_4d46bc3d-d4a1-48d7-963c-cab6d179e899" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeOperatingLeaseRenewalTerm"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_f9a20fae-f2fe-415b-aee6-39e93ba06b05" xlink:to="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_4d46bc3d-d4a1-48d7-963c-cab6d179e899" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_OperatingLeaseLeaseNotyetCommencedExpense_16155fb1-ed7c-490e-b4d4-3bb7fde81fcb" xlink:href="ftnt-20211231.xsd#ftnt_OperatingLeaseLeaseNotyetCommencedExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_f9a20fae-f2fe-415b-aee6-39e93ba06b05" xlink:to="loc_ftnt_OperatingLeaseLeaseNotyetCommencedExpense_16155fb1-ed7c-490e-b4d4-3bb7fde81fcb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1_2ef680ed-aa52-41dc-b3d2-6382d86ee664" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_f9a20fae-f2fe-415b-aee6-39e93ba06b05" xlink:to="loc_us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1_2ef680ed-aa52-41dc-b3d2-6382d86ee664" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_e12f7719-93d1-4bba-af5a-ec6f5f8efb2c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LesseeLeaseDescriptionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_f9a20fae-f2fe-415b-aee6-39e93ba06b05" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_e12f7719-93d1-4bba-af5a-ec6f5f8efb2c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_080b5e4b-addd-43d8-b7b1-732f3ce2deb1" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_e12f7719-93d1-4bba-af5a-ec6f5f8efb2c" xlink:to="loc_srt_RangeAxis_080b5e4b-addd-43d8-b7b1-732f3ce2deb1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_080b5e4b-addd-43d8-b7b1-732f3ce2deb1_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_080b5e4b-addd-43d8-b7b1-732f3ce2deb1" xlink:to="loc_srt_RangeMember_080b5e4b-addd-43d8-b7b1-732f3ce2deb1_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_693d907f-8c61-4a47-a79d-08711164db5d" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_080b5e4b-addd-43d8-b7b1-732f3ce2deb1" xlink:to="loc_srt_RangeMember_693d907f-8c61-4a47-a79d-08711164db5d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_29233093-ef28-4656-923a-a03efce18b7d" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_693d907f-8c61-4a47-a79d-08711164db5d" xlink:to="loc_srt_MinimumMember_29233093-ef28-4656-923a-a03efce18b7d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_f21193e9-ecf5-4798-9857-19b8e8da6a86" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_693d907f-8c61-4a47-a79d-08711164db5d" xlink:to="loc_srt_MaximumMember_f21193e9-ecf5-4798-9857-19b8e8da6a86" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/DebtNarrativeDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#DebtNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/DebtNarrativeDetails" xlink:type="extended" id="iff29fd1183f54e3db97cd2ea43d78019_DebtNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_d43b49bc-fa79-4719-8f08-17419d4875b0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_daa979ca-b718-4521-ab53-57e1335d728a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d43b49bc-fa79-4719-8f08-17419d4875b0" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_daa979ca-b718-4521-ab53-57e1335d728a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_7432d42b-972f-4548-842f-cca0601380c1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d43b49bc-fa79-4719-8f08-17419d4875b0" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_7432d42b-972f-4548-842f-cca0601380c1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestPayableCurrentAndNoncurrent_ce932901-408a-4717-ac38-94da9212f7a8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InterestPayableCurrentAndNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d43b49bc-fa79-4719-8f08-17419d4875b0" xlink:to="loc_us-gaap_InterestPayableCurrentAndNoncurrent_ce932901-408a-4717-ac38-94da9212f7a8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebt_fe4bf457-60ed-47f3-856b-a254d98bbd68" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InterestExpenseDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d43b49bc-fa79-4719-8f08-17419d4875b0" xlink:to="loc_us-gaap_InterestExpenseDebt_fe4bf457-60ed-47f3-856b-a254d98bbd68" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestPaidNet_dc43c362-9922-48bc-979d-fbb91d25e336" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InterestPaidNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d43b49bc-fa79-4719-8f08-17419d4875b0" xlink:to="loc_us-gaap_InterestPaidNet_dc43c362-9922-48bc-979d-fbb91d25e336" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestCostsCapitalized_db71d4f3-458f-4670-b7e6-e62107383a1b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_InterestCostsCapitalized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d43b49bc-fa79-4719-8f08-17419d4875b0" xlink:to="loc_us-gaap_InterestCostsCapitalized_db71d4f3-458f-4670-b7e6-e62107383a1b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtFairValue_41d7c36a-07a6-4a5c-9b38-16edd1bae099" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongTermDebtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d43b49bc-fa79-4719-8f08-17419d4875b0" xlink:to="loc_us-gaap_LongTermDebtFairValue_41d7c36a-07a6-4a5c-9b38-16edd1bae099" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_8bcca6ed-468c-409a-9588-7bca922f4d92" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_d43b49bc-fa79-4719-8f08-17419d4875b0" xlink:to="loc_us-gaap_DebtInstrumentTable_8bcca6ed-468c-409a-9588-7bca922f4d92" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_d9458e84-720c-4d23-a07e-4d10122c2e24" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_8bcca6ed-468c-409a-9588-7bca922f4d92" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_d9458e84-720c-4d23-a07e-4d10122c2e24" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_d9458e84-720c-4d23-a07e-4d10122c2e24_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_d9458e84-720c-4d23-a07e-4d10122c2e24" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_d9458e84-720c-4d23-a07e-4d10122c2e24_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_4ecef9cf-5e05-4592-ad55-fec50787c94d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_d9458e84-720c-4d23-a07e-4d10122c2e24" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_4ecef9cf-5e05-4592-ad55-fec50787c94d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorNotesMember_9f504880-7a01-47cd-974f-4af10496dc9a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_4ecef9cf-5e05-4592-ad55-fec50787c94d" xlink:to="loc_us-gaap_SeniorNotesMember_9f504880-7a01-47cd-974f-4af10496dc9a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_fabb935f-039e-4933-be78-ee863eec1bc5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_8bcca6ed-468c-409a-9588-7bca922f4d92" xlink:to="loc_us-gaap_DebtInstrumentAxis_fabb935f-039e-4933-be78-ee863eec1bc5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_fabb935f-039e-4933-be78-ee863eec1bc5_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_fabb935f-039e-4933-be78-ee863eec1bc5" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_fabb935f-039e-4933-be78-ee863eec1bc5_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_56254fdf-da7d-4038-8a87-dc6a0176c7ff" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_fabb935f-039e-4933-be78-ee863eec1bc5" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_56254fdf-da7d-4038-8a87-dc6a0176c7ff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_A2026SeniorNotesMember_333b9d6f-76f8-4ca4-8370-d258015a1ff3" xlink:href="ftnt-20211231.xsd#ftnt_A2026SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_56254fdf-da7d-4038-8a87-dc6a0176c7ff" xlink:to="loc_ftnt_A2026SeniorNotesMember_333b9d6f-76f8-4ca4-8370-d258015a1ff3" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_A2031SeniorNotesMember_11e775d1-12c9-41cc-ac45-1cf63c82ea32" xlink:href="ftnt-20211231.xsd#ftnt_A2031SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_56254fdf-da7d-4038-8a87-dc6a0176c7ff" xlink:to="loc_ftnt_A2031SeniorNotesMember_11e775d1-12c9-41cc-ac45-1cf63c82ea32" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_a4e8ab61-33ce-4673-b2a9-9d1d8ecf25b3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_8bcca6ed-468c-409a-9588-7bca922f4d92" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_a4e8ab61-33ce-4673-b2a9-9d1d8ecf25b3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_a4e8ab61-33ce-4673-b2a9-9d1d8ecf25b3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_a4e8ab61-33ce-4673-b2a9-9d1d8ecf25b3" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_a4e8ab61-33ce-4673-b2a9-9d1d8ecf25b3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_9b0ee593-b5b6-4fb1-a94f-cafc7672fd38" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_a4e8ab61-33ce-4673-b2a9-9d1d8ecf25b3" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_9b0ee593-b5b6-4fb1-a94f-cafc7672fd38" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_aeb40696-20f2-4b02-957a-5139bef95416" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_9b0ee593-b5b6-4fb1-a94f-cafc7672fd38" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_aeb40696-20f2-4b02-957a-5139bef95416" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/DebtSummaryofOutstandingDebtDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#DebtSummaryofOutstandingDebtDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/DebtSummaryofOutstandingDebtDetails" xlink:type="extended" id="ibeac4b26eb9740dca61107fa913be167_DebtSummaryofOutstandingDebtDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_5897a139-b016-4d0f-acad-956327608498" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_2a493cb3-2d3c-4e10-bba1-26066cf6bc5f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5897a139-b016-4d0f-acad-956327608498" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_2a493cb3-2d3c-4e10-bba1-26066cf6bc5f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_5bd48d19-edfd-4699-b42c-c9eee91aed5c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5897a139-b016-4d0f-acad-956327608498" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_5bd48d19-edfd-4699-b42c-c9eee91aed5c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_1003bc35-502d-4a4a-a233-eec150ec6bdd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5897a139-b016-4d0f-acad-956327608498" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_1003bc35-502d-4a4a-a233-eec150ec6bdd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_817634f2-654e-403e-87bc-d74d831f9871" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5897a139-b016-4d0f-acad-956327608498" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_817634f2-654e-403e-87bc-d74d831f9871" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_5bd6c8c2-a85a-431e-9ba6-4820f458e746" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5897a139-b016-4d0f-acad-956327608498" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_5bd6c8c2-a85a-431e-9ba6-4820f458e746" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_acff28b3-1565-4e12-bd5b-8e43930db340" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_5897a139-b016-4d0f-acad-956327608498" xlink:to="loc_us-gaap_DebtInstrumentTable_acff28b3-1565-4e12-bd5b-8e43930db340" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_7b09bfa7-ee69-46db-8c72-009e4fa88884" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_acff28b3-1565-4e12-bd5b-8e43930db340" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_7b09bfa7-ee69-46db-8c72-009e4fa88884" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_7b09bfa7-ee69-46db-8c72-009e4fa88884_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_7b09bfa7-ee69-46db-8c72-009e4fa88884" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_7b09bfa7-ee69-46db-8c72-009e4fa88884_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_14d1662f-bef4-46a7-9d67-3a2bc9652a7c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_7b09bfa7-ee69-46db-8c72-009e4fa88884" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_14d1662f-bef4-46a7-9d67-3a2bc9652a7c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorNotesMember_5bf1f079-8b5d-453d-92a9-09c8d1aa64b9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_14d1662f-bef4-46a7-9d67-3a2bc9652a7c" xlink:to="loc_us-gaap_SeniorNotesMember_5bf1f079-8b5d-453d-92a9-09c8d1aa64b9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_a759940d-aa3c-441f-bdd1-a37fde3ae7cf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_acff28b3-1565-4e12-bd5b-8e43930db340" xlink:to="loc_us-gaap_DebtInstrumentAxis_a759940d-aa3c-441f-bdd1-a37fde3ae7cf" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_a759940d-aa3c-441f-bdd1-a37fde3ae7cf_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_a759940d-aa3c-441f-bdd1-a37fde3ae7cf" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_a759940d-aa3c-441f-bdd1-a37fde3ae7cf_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_7191f328-e721-4781-a192-c91a2f836a71" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_a759940d-aa3c-441f-bdd1-a37fde3ae7cf" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_7191f328-e721-4781-a192-c91a2f836a71" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_A2026SeniorNotesMember_a7f242f9-e057-4398-ad17-a50b3cee5187" xlink:href="ftnt-20211231.xsd#ftnt_A2026SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_7191f328-e721-4781-a192-c91a2f836a71" xlink:to="loc_ftnt_A2026SeniorNotesMember_a7f242f9-e057-4398-ad17-a50b3cee5187" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_A2031SeniorNotesMember_109d8388-af22-4dc3-9982-3beef1b0fc0e" xlink:href="ftnt-20211231.xsd#ftnt_A2031SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_7191f328-e721-4781-a192-c91a2f836a71" xlink:to="loc_ftnt_A2031SeniorNotesMember_109d8388-af22-4dc3-9982-3beef1b0fc0e" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/EquityNarrativeDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#EquityNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/EquityNarrativeDetails" xlink:type="extended" id="id18dcd95bc0141f7af9aecfb41ba507b_EquityNarrativeDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_69852338-9e64-48bb-b3e1-f4899a8771d2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberofPrimaryStockIncentivePlans_82f52a8c-abd7-49a1-8b75-435075aff136" xlink:href="ftnt-20211231.xsd#ftnt_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberofPrimaryStockIncentivePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_69852338-9e64-48bb-b3e1-f4899a8771d2" xlink:to="loc_ftnt_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberofPrimaryStockIncentivePlans_82f52a8c-abd7-49a1-8b75-435075aff136" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberofSharesDeregistered_16b7ed3c-2bf2-4859-8c1d-02bbcf5e99cd" xlink:href="ftnt-20211231.xsd#ftnt_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberofSharesDeregistered"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_69852338-9e64-48bb-b3e1-f4899a8771d2" xlink:to="loc_ftnt_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberofSharesDeregistered_16b7ed3c-2bf2-4859-8c1d-02bbcf5e99cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_b4b9c0bc-76dc-4559-bbca-d50365c56b7a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_69852338-9e64-48bb-b3e1-f4899a8771d2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAuthorized_b4b9c0bc-76dc-4559-bbca-d50365c56b7a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_a350abfe-99cf-449a-97c7-5266c2dfe235" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_69852338-9e64-48bb-b3e1-f4899a8771d2" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_a350abfe-99cf-449a-97c7-5266c2dfe235" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardMinimumStockOwnershipPercentTriggeringEarlyAwardExpiration_9b2b7498-f838-4eba-9e57-096512b72554" xlink:href="ftnt-20211231.xsd#ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardMinimumStockOwnershipPercentTriggeringEarlyAwardExpiration"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_69852338-9e64-48bb-b3e1-f4899a8771d2" xlink:to="loc_ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardMinimumStockOwnershipPercentTriggeringEarlyAwardExpiration_9b2b7498-f838-4eba-9e57-096512b72554" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardMinimumExercisePriceAsPercentOfFairValueOnGrantDate_a9fa439a-6975-495c-910d-f0e914645b54" xlink:href="ftnt-20211231.xsd#ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardMinimumExercisePriceAsPercentOfFairValueOnGrantDate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_69852338-9e64-48bb-b3e1-f4899a8771d2" xlink:to="loc_ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardMinimumExercisePriceAsPercentOfFairValueOnGrantDate_a9fa439a-6975-495c-910d-f0e914645b54" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardExpirationPeriod_ba210a85-90fa-4571-b817-0332e272e626" xlink:href="ftnt-20211231.xsd#ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_69852338-9e64-48bb-b3e1-f4899a8771d2" xlink:to="loc_ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardExpirationPeriod_ba210a85-90fa-4571-b817-0332e272e626" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumStockOwnershipPercentForOptionsGrantedToIndividuals_cf9b63dd-ec9c-40da-a777-724e5e53e3fc" xlink:href="ftnt-20211231.xsd#ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumStockOwnershipPercentForOptionsGrantedToIndividuals"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_69852338-9e64-48bb-b3e1-f4899a8771d2" xlink:to="loc_ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumStockOwnershipPercentForOptionsGrantedToIndividuals_cf9b63dd-ec9c-40da-a777-724e5e53e3fc" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_2c492ccf-8bee-4f88-b328-00551723d15a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_69852338-9e64-48bb-b3e1-f4899a8771d2" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardExpirationPeriod_2c492ccf-8bee-4f88-b328-00551723d15a" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_debda711-9aa9-4012-81d2-32f4e5e59ee0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_69852338-9e64-48bb-b3e1-f4899a8771d2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_debda711-9aa9-4012-81d2-32f4e5e59ee0" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_7a5a5349-db3c-4c7a-a2fa-6b2411a98148" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_69852338-9e64-48bb-b3e1-f4899a8771d2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_7a5a5349-db3c-4c7a-a2fa-6b2411a98148" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumEmployeeSubscriptionRate_1112897e-c7dd-4db9-a7b2-c071fc64034c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumEmployeeSubscriptionRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_69852338-9e64-48bb-b3e1-f4899a8771d2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardMaximumEmployeeSubscriptionRate_1112897e-c7dd-4db9-a7b2-c071fc64034c" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_PurchasePriceOfCommonStockAsPercentageOfLowerOfFairMarketValueOfCommonStockOnFirstDayOfOfferingPeriodOrLastDayOfPurchasePeriod_081e4bc8-3652-4769-ac70-9fc8fc399451" xlink:href="ftnt-20211231.xsd#ftnt_PurchasePriceOfCommonStockAsPercentageOfLowerOfFairMarketValueOfCommonStockOnFirstDayOfOfferingPeriodOrLastDayOfPurchasePeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_69852338-9e64-48bb-b3e1-f4899a8771d2" xlink:to="loc_ftnt_PurchasePriceOfCommonStockAsPercentageOfLowerOfFairMarketValueOfCommonStockOnFirstDayOfOfferingPeriodOrLastDayOfPurchasePeriod_081e4bc8-3652-4769-ac70-9fc8fc399451" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_EmployeeStockPurchasePlanPeriodicPurchaseLimit_e475d214-8b58-4a36-b6fa-748ee5644b80" xlink:href="ftnt-20211231.xsd#ftnt_EmployeeStockPurchasePlanPeriodicPurchaseLimit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_69852338-9e64-48bb-b3e1-f4899a8771d2" xlink:to="loc_ftnt_EmployeeStockPurchasePlanPeriodicPurchaseLimit_e475d214-8b58-4a36-b6fa-748ee5644b80" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_EmployeeStockPurchasePlanAnnualPurchaseLimit_c85ce5da-a709-4e4a-9a0a-e71aacbee8ef" xlink:href="ftnt-20211231.xsd#ftnt_EmployeeStockPurchasePlanAnnualPurchaseLimit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_69852338-9e64-48bb-b3e1-f4899a8771d2" xlink:to="loc_ftnt_EmployeeStockPurchasePlanAnnualPurchaseLimit_c85ce5da-a709-4e4a-9a0a-e71aacbee8ef" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_a920b5f4-0097-4e00-a9ff-c4105b09505b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_69852338-9e64-48bb-b3e1-f4899a8771d2" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_a920b5f4-0097-4e00-a9ff-c4105b09505b" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_34a30643-aace-45b1-9d1f-34e1f44d3939" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_69852338-9e64-48bb-b3e1-f4899a8771d2" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_34a30643-aace-45b1-9d1f-34e1f44d3939" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_43bf87dc-6d89-4c18-91db-dad5b36d62a8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_69852338-9e64-48bb-b3e1-f4899a8771d2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_43bf87dc-6d89-4c18-91db-dad5b36d62a8" xlink:type="arc" order="17"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_3c6b4bab-ab1f-48f7-b38a-546847fcc39b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_69852338-9e64-48bb-b3e1-f4899a8771d2" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_3c6b4bab-ab1f-48f7-b38a-546847fcc39b" xlink:type="arc" order="18"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_aa762fcd-6864-421c-807f-cb9c080ac773" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_69852338-9e64-48bb-b3e1-f4899a8771d2" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_aa762fcd-6864-421c-807f-cb9c080ac773" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameAxis_c99e0c62-91ac-4f48-a448-8073c9a99e76" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_aa762fcd-6864-421c-807f-cb9c080ac773" xlink:to="loc_us-gaap_PlanNameAxis_c99e0c62-91ac-4f48-a448-8073c9a99e76" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_c99e0c62-91ac-4f48-a448-8073c9a99e76_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PlanNameAxis_c99e0c62-91ac-4f48-a448-8073c9a99e76" xlink:to="loc_us-gaap_PlanNameDomain_c99e0c62-91ac-4f48-a448-8073c9a99e76_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PlanNameDomain_1ad75259-6be9-4323-876b-5905b9c8392e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PlanNameAxis_c99e0c62-91ac-4f48-a448-8073c9a99e76" xlink:to="loc_us-gaap_PlanNameDomain_1ad75259-6be9-4323-876b-5905b9c8392e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_StockPlan2009Member_f07d3349-4c68-4ae7-af1c-17d1c8650d51" xlink:href="ftnt-20211231.xsd#ftnt_StockPlan2009Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PlanNameDomain_1ad75259-6be9-4323-876b-5905b9c8392e" xlink:to="loc_ftnt_StockPlan2009Member_f07d3349-4c68-4ae7-af1c-17d1c8650d51" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualAxis_b8ba4c72-2f0d-4e72-b349-ed4dc495c118" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_TitleOfIndividualAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_aa762fcd-6864-421c-807f-cb9c080ac773" xlink:to="loc_srt_TitleOfIndividualAxis_b8ba4c72-2f0d-4e72-b349-ed4dc495c118" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_b8ba4c72-2f0d-4e72-b349-ed4dc495c118_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_TitleOfIndividualAxis_b8ba4c72-2f0d-4e72-b349-ed4dc495c118" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_b8ba4c72-2f0d-4e72-b349-ed4dc495c118_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_983e2ce4-1dc7-4353-98bc-9660c93c75c8" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_TitleOfIndividualAxis_b8ba4c72-2f0d-4e72-b349-ed4dc495c118" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_983e2ce4-1dc7-4353-98bc-9660c93c75c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_IndividualOwning10PercentOrMoreOfStockMember_5dfaf9be-5d87-4f8a-b26d-389ae1341030" xlink:href="ftnt-20211231.xsd#ftnt_IndividualOwning10PercentOrMoreOfStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_983e2ce4-1dc7-4353-98bc-9660c93c75c8" xlink:to="loc_ftnt_IndividualOwning10PercentOrMoreOfStockMember_5dfaf9be-5d87-4f8a-b26d-389ae1341030" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_EmployeeMember_926ee841-fe60-4c00-8c6c-abf80793a663" xlink:href="ftnt-20211231.xsd#ftnt_EmployeeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_983e2ce4-1dc7-4353-98bc-9660c93c75c8" xlink:to="loc_ftnt_EmployeeMember_926ee841-fe60-4c00-8c6c-abf80793a663" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_DirectorsAndOtherServiceProvidersMember_a904a2bc-69a5-48c8-aab7-d4901b3cd243" xlink:href="ftnt-20211231.xsd#ftnt_DirectorsAndOtherServiceProvidersMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_983e2ce4-1dc7-4353-98bc-9660c93c75c8" xlink:to="loc_ftnt_DirectorsAndOtherServiceProvidersMember_a904a2bc-69a5-48c8-aab7-d4901b3cd243" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_IndividualOwningLessThan10PercentOfStockMember_30835727-c9e4-4ba0-b63e-70431102e38c" xlink:href="ftnt-20211231.xsd#ftnt_IndividualOwningLessThan10PercentOfStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_983e2ce4-1dc7-4353-98bc-9660c93c75c8" xlink:to="loc_ftnt_IndividualOwningLessThan10PercentOfStockMember_30835727-c9e4-4ba0-b63e-70431102e38c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_da0871cb-4eeb-4ecf-b4d4-ba4f4c395cba" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_aa762fcd-6864-421c-807f-cb9c080ac773" xlink:to="loc_us-gaap_AwardTypeAxis_da0871cb-4eeb-4ecf-b4d4-ba4f4c395cba" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_da0871cb-4eeb-4ecf-b4d4-ba4f4c395cba_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_da0871cb-4eeb-4ecf-b4d4-ba4f4c395cba" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_da0871cb-4eeb-4ecf-b4d4-ba4f4c395cba_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2c65ddeb-e64c-480a-95c3-72e244d4e337" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_da0871cb-4eeb-4ecf-b4d4-ba4f4c395cba" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2c65ddeb-e64c-480a-95c3-72e244d4e337" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_8ff39a8f-1310-4290-a7f6-b9c7f64d5b74" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2c65ddeb-e64c-480a-95c3-72e244d4e337" xlink:to="loc_us-gaap_EmployeeStockOptionMember_8ff39a8f-1310-4290-a7f6-b9c7f64d5b74" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_StockOptionsNonqualifyingMember_3953e34e-5e8d-4bea-94de-113572582198" xlink:href="ftnt-20211231.xsd#ftnt_StockOptionsNonqualifyingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2c65ddeb-e64c-480a-95c3-72e244d4e337" xlink:to="loc_ftnt_StockOptionsNonqualifyingMember_3953e34e-5e8d-4bea-94de-113572582198" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_060a24f8-c73e-4cab-8f67-3fec8642c702" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2c65ddeb-e64c-480a-95c3-72e244d4e337" xlink:to="loc_us-gaap_EmployeeStockMember_060a24f8-c73e-4cab-8f67-3fec8642c702" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_RestrictedStockUnitsAndOptionsMember_83bf8d35-044b-4529-af4f-451abbee62f1" xlink:href="ftnt-20211231.xsd#ftnt_RestrictedStockUnitsAndOptionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_2c65ddeb-e64c-480a-95c3-72e244d4e337" xlink:to="loc_ftnt_RestrictedStockUnitsAndOptionsMember_83bf8d35-044b-4529-af4f-451abbee62f1" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/EquityRestrictedStockUnitsActivityDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#EquityRestrictedStockUnitsActivityDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/EquityRestrictedStockUnitsActivityDetails" xlink:type="extended" id="i879e7886c57f4a7ab04db08c7d8826dd_EquityRestrictedStockUnitsActivityDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b3a604d9-e997-43b3-be6b-50d0a90555b5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_03355dc6-953a-48e5-a4dc-4610ea40c397" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b3a604d9-e997-43b3-be6b-50d0a90555b5" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_03355dc6-953a-48e5-a4dc-4610ea40c397" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_72176d1a-075c-4174-84f6-42bce2489f26" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_03355dc6-953a-48e5-a4dc-4610ea40c397" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_72176d1a-075c-4174-84f6-42bce2489f26" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_a0db64c0-5e23-4a50-b95e-4f8b738e02de" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_03355dc6-953a-48e5-a4dc-4610ea40c397" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_a0db64c0-5e23-4a50-b95e-4f8b738e02de" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_49d5d1a5-0050-4b8d-9088-75283a0dd267" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_03355dc6-953a-48e5-a4dc-4610ea40c397" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_49d5d1a5-0050-4b8d-9088-75283a0dd267" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_716d9381-5237-4a08-a487-a8c00af136b2" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_03355dc6-953a-48e5-a4dc-4610ea40c397" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_716d9381-5237-4a08-a487-a8c00af136b2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_c3e9a525-f13c-4450-bc02-d2b60a372c0a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_8878aa20-76b8-4bb3-aac7-651583c9713c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b3a604d9-e997-43b3-be6b-50d0a90555b5" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_8878aa20-76b8-4bb3-aac7-651583c9713c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_4197de72-3fda-4113-a6cf-76e50ff932db" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_8878aa20-76b8-4bb3-aac7-651583c9713c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_4197de72-3fda-4113-a6cf-76e50ff932db" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_80e2c83b-b1d6-42b8-bf7b-0edd6540a9ad" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_8878aa20-76b8-4bb3-aac7-651583c9713c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_80e2c83b-b1d6-42b8-bf7b-0edd6540a9ad" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_7b995973-a2e0-4134-895a-69ca71779e95" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_8878aa20-76b8-4bb3-aac7-651583c9713c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_7b995973-a2e0-4134-895a-69ca71779e95" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_7925ad1f-ba15-43a9-8231-9a20de3fb7b5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_8878aa20-76b8-4bb3-aac7-651583c9713c" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_7925ad1f-ba15-43a9-8231-9a20de3fb7b5" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_9c19dced-25f6-4158-9164-2ad2f691917d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_25b2decc-2784-4634-8930-f4a066ff2d29" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b3a604d9-e997-43b3-be6b-50d0a90555b5" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_25b2decc-2784-4634-8930-f4a066ff2d29" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_e0e075e0-cc94-4fc1-b348-f38dd70175ba" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b3a604d9-e997-43b3-be6b-50d0a90555b5" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_e0e075e0-cc94-4fc1-b348-f38dd70175ba" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_e1f33de4-cffa-448d-87af-0b62bc1abaec" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b3a604d9-e997-43b3-be6b-50d0a90555b5" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_e1f33de4-cffa-448d-87af-0b62bc1abaec" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_039622c3-04dd-4761-8da3-70db3405697a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_e1f33de4-cffa-448d-87af-0b62bc1abaec" xlink:to="loc_us-gaap_AwardTypeAxis_039622c3-04dd-4761-8da3-70db3405697a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_039622c3-04dd-4761-8da3-70db3405697a_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_039622c3-04dd-4761-8da3-70db3405697a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_039622c3-04dd-4761-8da3-70db3405697a_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8c51d8d1-c6e7-48f3-af53-45185faa20bf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_039622c3-04dd-4761-8da3-70db3405697a" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8c51d8d1-c6e7-48f3-af53-45185faa20bf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_f28156bd-331d-421f-808a-1964ce5a566e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_8c51d8d1-c6e7-48f3-af53-45185faa20bf" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_f28156bd-331d-421f-808a-1964ce5a566e" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/EquityScheduleofSharebasedCompensationSharesWithheldforTaxesDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#EquityScheduleofSharebasedCompensationSharesWithheldforTaxesDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/EquityScheduleofSharebasedCompensationSharesWithheldforTaxesDetails" xlink:type="extended" id="ife665640a7a54cf8b3fb19a444aa11ab_EquityScheduleofSharebasedCompensationSharesWithheldforTaxesDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_84a6953e-2aef-49f5-a450-7661a6a5b298" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_7e35b776-36d6-47b7-a98d-0a28ac779900" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_84a6953e-2aef-49f5-a450-7661a6a5b298" xlink:to="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_7e35b776-36d6-47b7-a98d-0a28ac779900" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_0b9ebf96-838f-48af-8e40-2fb948e4f59d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_84a6953e-2aef-49f5-a450-7661a6a5b298" xlink:to="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_0b9ebf96-838f-48af-8e40-2fb948e4f59d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_be0272b6-5eea-4071-9d06-7a9bb489d581" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_84a6953e-2aef-49f5-a450-7661a6a5b298" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_be0272b6-5eea-4071-9d06-7a9bb489d581" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_15243b85-65f1-40d0-b8dc-4e2cd29e0a9b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_be0272b6-5eea-4071-9d06-7a9bb489d581" xlink:to="loc_us-gaap_AwardTypeAxis_15243b85-65f1-40d0-b8dc-4e2cd29e0a9b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_15243b85-65f1-40d0-b8dc-4e2cd29e0a9b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_15243b85-65f1-40d0-b8dc-4e2cd29e0a9b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_15243b85-65f1-40d0-b8dc-4e2cd29e0a9b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c464fff9-29d1-473d-8515-325da00332d6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_15243b85-65f1-40d0-b8dc-4e2cd29e0a9b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c464fff9-29d1-473d-8515-325da00332d6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_2c72442b-528a-4146-a518-6a03afeb8145" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c464fff9-29d1-473d-8515-325da00332d6" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_2c72442b-528a-4146-a518-6a03afeb8145" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/EquityScheduleofSharebasedPaymentAwardStockOptionsValuationAssumptionsDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#EquityScheduleofSharebasedPaymentAwardStockOptionsValuationAssumptionsDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/EquityScheduleofSharebasedPaymentAwardStockOptionsValuationAssumptionsDetails" xlink:type="extended" id="i6dff97b5259f42a9b8a072397bd60ba8_EquityScheduleofSharebasedPaymentAwardStockOptionsValuationAssumptionsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_316f8123-e57f-4157-98c7-b69dc9d162a1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_4bac59ee-0dde-4ab3-b184-31922b824bff" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_316f8123-e57f-4157-98c7-b69dc9d162a1" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_4bac59ee-0dde-4ab3-b184-31922b824bff" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_ddca3950-0852-425e-b1f0-a6e9672dfc4c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_316f8123-e57f-4157-98c7-b69dc9d162a1" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_ddca3950-0852-425e-b1f0-a6e9672dfc4c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_3eda0334-b5ed-4eff-add5-525c9f4174a1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_316f8123-e57f-4157-98c7-b69dc9d162a1" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_3eda0334-b5ed-4eff-add5-525c9f4174a1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_f186ab2d-dae9-4464-a035-c2431e8dcf4f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_316f8123-e57f-4157-98c7-b69dc9d162a1" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_f186ab2d-dae9-4464-a035-c2431e8dcf4f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_57906e21-3896-4a30-8067-30a3a09c2d03" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_316f8123-e57f-4157-98c7-b69dc9d162a1" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_57906e21-3896-4a30-8067-30a3a09c2d03" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_f0193275-669f-4010-b796-41ef1b93f2fb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_57906e21-3896-4a30-8067-30a3a09c2d03" xlink:to="loc_us-gaap_AwardTypeAxis_f0193275-669f-4010-b796-41ef1b93f2fb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f0193275-669f-4010-b796-41ef1b93f2fb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_f0193275-669f-4010-b796-41ef1b93f2fb" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f0193275-669f-4010-b796-41ef1b93f2fb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_40659ae5-19d2-4129-b884-6b2dad5186d9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_f0193275-669f-4010-b796-41ef1b93f2fb" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_40659ae5-19d2-4129-b884-6b2dad5186d9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_aaf14d70-d9c7-4a2c-bc2f-9945138fa89b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_40659ae5-19d2-4129-b884-6b2dad5186d9" xlink:to="loc_us-gaap_EmployeeStockOptionMember_aaf14d70-d9c7-4a2c-bc2f-9945138fa89b" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/EquityScheduleofSharebasedCompensationStockOptionsActivityDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#EquityScheduleofSharebasedCompensationStockOptionsActivityDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/EquityScheduleofSharebasedCompensationStockOptionsActivityDetails" xlink:type="extended" id="i0c0af7648295494a89333e1ff54aaee9_EquityScheduleofSharebasedCompensationStockOptionsActivityDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d56c7c01-0e06-420f-aaeb-efb15ae0ae49" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_e0d5598b-ca9a-4618-88aa-eb9b7aa98951" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d56c7c01-0e06-420f-aaeb-efb15ae0ae49" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_e0d5598b-ca9a-4618-88aa-eb9b7aa98951" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_1010e4bf-5f33-4021-beee-6e98ca443d47" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_e0d5598b-ca9a-4618-88aa-eb9b7aa98951" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_1010e4bf-5f33-4021-beee-6e98ca443d47" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_8807571c-7cbc-40d9-a345-eca2e3ca0e8a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_e0d5598b-ca9a-4618-88aa-eb9b7aa98951" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_8807571c-7cbc-40d9-a345-eca2e3ca0e8a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_7bf30b52-5385-47b1-8de1-ef53cb180831" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_e0d5598b-ca9a-4618-88aa-eb9b7aa98951" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_7bf30b52-5385-47b1-8de1-ef53cb180831" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_3bd46753-86f3-4011-88d0-b926fbca0ea1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_e0d5598b-ca9a-4618-88aa-eb9b7aa98951" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_3bd46753-86f3-4011-88d0-b926fbca0ea1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_e9589113-6fff-4616-a0b0-63e6bfd7322a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_6792602b-0569-4b93-9cc2-814b1022ef56" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d56c7c01-0e06-420f-aaeb-efb15ae0ae49" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_6792602b-0569-4b93-9cc2-814b1022ef56" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_f7ec2504-b58c-40eb-9bd8-1b0ab20231c1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_6792602b-0569-4b93-9cc2-814b1022ef56" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_f7ec2504-b58c-40eb-9bd8-1b0ab20231c1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_9a5e3b5d-6078-4257-a711-4ecf73b4d223" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_6792602b-0569-4b93-9cc2-814b1022ef56" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_9a5e3b5d-6078-4257-a711-4ecf73b4d223" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_14282a6e-6d44-47c0-934a-bcca11bd887f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_6792602b-0569-4b93-9cc2-814b1022ef56" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_14282a6e-6d44-47c0-934a-bcca11bd887f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_66446383-b4ac-4207-bc20-95bc7461ec22" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_6792602b-0569-4b93-9cc2-814b1022ef56" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_66446383-b4ac-4207-bc20-95bc7461ec22" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_9df31c95-cf23-4863-8d27-2f4e3be08f75" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_9a7f60bf-75e6-42bc-a92a-963dc5327c47" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d56c7c01-0e06-420f-aaeb-efb15ae0ae49" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_9a7f60bf-75e6-42bc-a92a-963dc5327c47" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_3ec99f39-572a-4713-ba63-bbe5f38fc836" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_9a7f60bf-75e6-42bc-a92a-963dc5327c47" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_3ec99f39-572a-4713-ba63-bbe5f38fc836" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_9241a0b7-149e-4e33-a9b1-666ae34190e8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_9a7f60bf-75e6-42bc-a92a-963dc5327c47" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_9241a0b7-149e-4e33-a9b1-666ae34190e8" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_5f26cdab-b62d-42c2-b41f-05e30b5fdefe" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_9a7f60bf-75e6-42bc-a92a-963dc5327c47" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_5f26cdab-b62d-42c2-b41f-05e30b5fdefe" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_65d78335-d651-4375-8226-92acb7620a5d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_9a7f60bf-75e6-42bc-a92a-963dc5327c47" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_65d78335-d651-4375-8226-92acb7620a5d" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_ae7d5864-55c1-4a45-ae02-1357bedc64fd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_9a7f60bf-75e6-42bc-a92a-963dc5327c47" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_ae7d5864-55c1-4a45-ae02-1357bedc64fd" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_2aacda35-777c-4c11-9a21-4f7beb6e99de" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_9a7f60bf-75e6-42bc-a92a-963dc5327c47" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_2aacda35-777c-4c11-9a21-4f7beb6e99de" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_17344981-e5ef-4156-8174-f082533c29e7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_9a7f60bf-75e6-42bc-a92a-963dc5327c47" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_17344981-e5ef-4156-8174-f082533c29e7" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_1f12930b-de94-4c0d-84b6-4834a65e349f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_9a7f60bf-75e6-42bc-a92a-963dc5327c47" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_1f12930b-de94-4c0d-84b6-4834a65e349f" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_3948291c-0185-478e-953d-4244fd09892c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_9a7f60bf-75e6-42bc-a92a-963dc5327c47" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_3948291c-0185-478e-953d-4244fd09892c" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_022dda8b-dec2-4aac-8b0b-61efcd2b3f35" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_9a7f60bf-75e6-42bc-a92a-963dc5327c47" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_022dda8b-dec2-4aac-8b0b-61efcd2b3f35" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4192ecb1-ddb3-489a-a2ea-c6e0ae990b2d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_d56c7c01-0e06-420f-aaeb-efb15ae0ae49" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4192ecb1-ddb3-489a-a2ea-c6e0ae990b2d" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_f63c11bf-d326-4e52-83d4-658eaad23a52" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_4192ecb1-ddb3-489a-a2ea-c6e0ae990b2d" xlink:to="loc_us-gaap_AwardTypeAxis_f63c11bf-d326-4e52-83d4-658eaad23a52" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f63c11bf-d326-4e52-83d4-658eaad23a52_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_f63c11bf-d326-4e52-83d4-658eaad23a52" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f63c11bf-d326-4e52-83d4-658eaad23a52_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c1c82d50-2811-4cfd-ba10-abc2b151c38d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_f63c11bf-d326-4e52-83d4-658eaad23a52" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c1c82d50-2811-4cfd-ba10-abc2b151c38d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_c295afee-f776-4636-918e-d6b8c13c2d91" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_c1c82d50-2811-4cfd-ba10-abc2b151c38d" xlink:to="loc_us-gaap_EmployeeStockOptionMember_c295afee-f776-4636-918e-d6b8c13c2d91" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/EquityScheduleofSharebasedCompensationStockOptionsActivityAdditionalInformationDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#EquityScheduleofSharebasedCompensationStockOptionsActivityAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/EquityScheduleofSharebasedCompensationStockOptionsActivityAdditionalInformationDetails" xlink:type="extended" id="idf79a740ac3746678458c38ecf11a206_EquityScheduleofSharebasedCompensationStockOptionsActivityAdditionalInformationDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_1f28593d-017d-4f43-ac43-771c0f5c1263" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_d03d92d4-0d2b-46d2-9b77-5e51b8a9e027" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_1f28593d-017d-4f43-ac43-771c0f5c1263" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_d03d92d4-0d2b-46d2-9b77-5e51b8a9e027" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_9787855e-744a-4d8e-b0cd-e61c0d3b62c4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_1f28593d-017d-4f43-ac43-771c0f5c1263" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_9787855e-744a-4d8e-b0cd-e61c0d3b62c4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_9dc7ff08-f726-49a3-a92d-db01df6b0f9c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_1f28593d-017d-4f43-ac43-771c0f5c1263" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_9dc7ff08-f726-49a3-a92d-db01df6b0f9c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_e0fc9420-8eb1-4ebd-9147-40aee33a532f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_1f28593d-017d-4f43-ac43-771c0f5c1263" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_e0fc9420-8eb1-4ebd-9147-40aee33a532f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_77ce3c65-6bb9-47f1-b16c-9ef65f49573f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_e0fc9420-8eb1-4ebd-9147-40aee33a532f" xlink:to="loc_us-gaap_AwardTypeAxis_77ce3c65-6bb9-47f1-b16c-9ef65f49573f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_77ce3c65-6bb9-47f1-b16c-9ef65f49573f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_77ce3c65-6bb9-47f1-b16c-9ef65f49573f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_77ce3c65-6bb9-47f1-b16c-9ef65f49573f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4f26786f-c17d-4ec4-80b6-f5651a82f578" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_77ce3c65-6bb9-47f1-b16c-9ef65f49573f" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4f26786f-c17d-4ec4-80b6-f5651a82f578" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_8b5ec086-e5c0-4598-be07-2dff4f4a42e4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4f26786f-c17d-4ec4-80b6-f5651a82f578" xlink:to="loc_us-gaap_EmployeeStockOptionMember_8b5ec086-e5c0-4598-be07-2dff4f4a42e4" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/EquityRangeofOptionsDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#EquityRangeofOptionsDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/EquityRangeofOptionsDetails" xlink:type="extended" id="i9ac7a8ff2fd84513bb945594e90fb8a5_EquityRangeofOptionsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems_d6475955-3642-43f8-ad6e-34cfb5b61c59" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit_6c4c39cb-12d5-4b00-8c9b-22b976903172" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems_d6475955-3642-43f8-ad6e-34cfb5b61c59" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLowerRangeLimit_6c4c39cb-12d5-4b00-8c9b-22b976903172" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit_f4351791-386f-4a15-abe6-f94b87cb5578" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems_d6475955-3642-43f8-ad6e-34cfb5b61c59" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeUpperRangeLimit_f4351791-386f-4a15-abe6-f94b87cb5578" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions_724de469-0bd3-4dba-b3a5-d94b68b2edcb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems_d6475955-3642-43f8-ad6e-34cfb5b61c59" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfOutstandingOptions_724de469-0bd3-4dba-b3a5-d94b68b2edcb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageRemainingContractualTerm2_e1520120-42e8-47bc-a726-c875b2323dbf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageRemainingContractualTerm2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems_d6475955-3642-43f8-ad6e-34cfb5b61c59" xlink:to="loc_us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageRemainingContractualTerm2_e1520120-42e8-47bc-a726-c875b2323dbf" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1_313bdb43-9941-4828-aaf7-c6855d80fe6c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems_d6475955-3642-43f8-ad6e-34cfb5b61c59" xlink:to="loc_us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeOutstandingOptionsWeightedAverageExercisePriceBeginningBalance1_313bdb43-9941-4828-aaf7-c6855d80fe6c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions_6e8e9ad9-5530-437a-bf33-421b24589b49" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems_d6475955-3642-43f8-ad6e-34cfb5b61c59" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeNumberOfExercisableOptions_6e8e9ad9-5530-437a-bf33-421b24589b49" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageExercisePrice1_dcc3ce25-2e16-40bb-81ac-5789eddc8c8e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageExercisePrice1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems_d6475955-3642-43f8-ad6e-34cfb5b61c59" xlink:to="loc_us-gaap_SharebasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeExercisableOptionsWeightedAverageExercisePrice1_dcc3ce25-2e16-40bb-81ac-5789eddc8c8e" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeTable_e844f1ab-2559-4fa7-849f-2f36682c6e17" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeLineItems_d6475955-3642-43f8-ad6e-34cfb5b61c59" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeTable_e844f1ab-2559-4fa7-849f-2f36682c6e17" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis_97f32e6b-9a89-48f5-9a5a-7c719e2a5f11" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeTable_e844f1ab-2559-4fa7-849f-2f36682c6e17" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis_97f32e6b-9a89-48f5-9a5a-7c719e2a5f11" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_97f32e6b-9a89-48f5-9a5a-7c719e2a5f11_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis_97f32e6b-9a89-48f5-9a5a-7c719e2a5f11" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_97f32e6b-9a89-48f5-9a5a-7c719e2a5f11_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_72a4e592-13d5-4893-b6a0-180c3f7976a7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansByExercisePriceRangeAxis_97f32e6b-9a89-48f5-9a5a-7c719e2a5f11" xlink:to="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_72a4e592-13d5-4893-b6a0-180c3f7976a7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_PriceRangeOneMember_f1e63523-ae65-416a-9205-af62db2aa866" xlink:href="ftnt-20211231.xsd#ftnt_PriceRangeOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_72a4e592-13d5-4893-b6a0-180c3f7976a7" xlink:to="loc_ftnt_PriceRangeOneMember_f1e63523-ae65-416a-9205-af62db2aa866" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_PriceRangeTwoMember_b9555393-0769-4833-9c99-57064d95d702" xlink:href="ftnt-20211231.xsd#ftnt_PriceRangeTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_72a4e592-13d5-4893-b6a0-180c3f7976a7" xlink:to="loc_ftnt_PriceRangeTwoMember_b9555393-0769-4833-9c99-57064d95d702" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_PriceRangeThreeMember_8f174eba-d402-4bbe-b201-ed413a3ebe1d" xlink:href="ftnt-20211231.xsd#ftnt_PriceRangeThreeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_72a4e592-13d5-4893-b6a0-180c3f7976a7" xlink:to="loc_ftnt_PriceRangeThreeMember_8f174eba-d402-4bbe-b201-ed413a3ebe1d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_PriceRangeFourMember_1a03190c-38d5-4486-afd6-9423a4705158" xlink:href="ftnt-20211231.xsd#ftnt_PriceRangeFourMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationSharesAuthorizedUnderStockOptionPlansExercisePriceRangeDomain_72a4e592-13d5-4893-b6a0-180c3f7976a7" xlink:to="loc_ftnt_PriceRangeFourMember_1a03190c-38d5-4486-afd6-9423a4705158" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/EquityAdditionalInformationRelatedToESPPDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#EquityAdditionalInformationRelatedToESPPDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/EquityAdditionalInformationRelatedToESPPDetails" xlink:type="extended" id="i262da405424044039c1e9c499a5b6c36_EquityAdditionalInformationRelatedToESPPDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b5e217fc-7886-41eb-b95b-2ce41ab5faa4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_eae518a8-a01e-46dc-b737-b10ece0b84eb" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b5e217fc-7886-41eb-b95b-2ce41ab5faa4" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_eae518a8-a01e-46dc-b737-b10ece0b84eb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsIssuedWeightedAveragePricePerShare_bb02067f-3e4a-45e7-9797-f4e8e1e33b8b" xlink:href="ftnt-20211231.xsd#ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsIssuedWeightedAveragePricePerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b5e217fc-7886-41eb-b95b-2ce41ab5faa4" xlink:to="loc_ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsIssuedWeightedAveragePricePerShare_bb02067f-3e4a-45e7-9797-f4e8e1e33b8b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_8297959b-0501-4b49-be58-a94d6ad15516" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_b5e217fc-7886-41eb-b95b-2ce41ab5faa4" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_8297959b-0501-4b49-be58-a94d6ad15516" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_9564e306-085a-4366-91bc-eefd29e8326b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_8297959b-0501-4b49-be58-a94d6ad15516" xlink:to="loc_us-gaap_AwardTypeAxis_9564e306-085a-4366-91bc-eefd29e8326b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_9564e306-085a-4366-91bc-eefd29e8326b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_9564e306-085a-4366-91bc-eefd29e8326b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_9564e306-085a-4366-91bc-eefd29e8326b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_add38461-46d6-42a3-893a-2dc7d1552a38" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_9564e306-085a-4366-91bc-eefd29e8326b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_add38461-46d6-42a3-893a-2dc7d1552a38" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_24c73aff-48ce-4ef6-9f6a-db7e50394f82" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_add38461-46d6-42a3-893a-2dc7d1552a38" xlink:to="loc_us-gaap_EmployeeStockMember_24c73aff-48ce-4ef6-9f6a-db7e50394f82" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/EquityScheduleofSharesReservedforFutureIssuanceDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#EquityScheduleofSharesReservedforFutureIssuanceDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/EquityScheduleofSharesReservedforFutureIssuanceDetails" xlink:type="extended" id="ib0e4c71656554ca4acd7294ae011068e_EquityScheduleofSharesReservedforFutureIssuanceDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_84079d71-e2ab-43fc-92b5-add604e4e74f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_06d0d4d0-06af-4b8e-881d-839446446e6b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockCapitalSharesReservedForFutureIssuance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_84079d71-e2ab-43fc-92b5-add604e4e74f" xlink:to="loc_us-gaap_CommonStockCapitalSharesReservedForFutureIssuance_06d0d4d0-06af-4b8e-881d-839446446e6b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_261b6a90-69b7-4043-9309-8c1ac8f02c8c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_84079d71-e2ab-43fc-92b5-add604e4e74f" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_261b6a90-69b7-4043-9309-8c1ac8f02c8c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_be62b5bb-da1e-45a4-9710-40c8b6a05162" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_261b6a90-69b7-4043-9309-8c1ac8f02c8c" xlink:to="loc_us-gaap_AwardTypeAxis_be62b5bb-da1e-45a4-9710-40c8b6a05162" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_be62b5bb-da1e-45a4-9710-40c8b6a05162_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_be62b5bb-da1e-45a4-9710-40c8b6a05162" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_be62b5bb-da1e-45a4-9710-40c8b6a05162_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6dda3b6f-3fa5-495a-884b-0bca3fb8d289" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_be62b5bb-da1e-45a4-9710-40c8b6a05162" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6dda3b6f-3fa5-495a-884b-0bca3fb8d289" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ReservedForFutureOptionRestrictedStockUnitAndOtherEquityAwardGrantsMember_1bbd92d1-36bc-46bf-8ab6-f7d7d47e37d9" xlink:href="ftnt-20211231.xsd#ftnt_ReservedForFutureOptionRestrictedStockUnitAndOtherEquityAwardGrantsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6dda3b6f-3fa5-495a-884b-0bca3fb8d289" xlink:to="loc_ftnt_ReservedForFutureOptionRestrictedStockUnitAndOtherEquityAwardGrantsMember_1bbd92d1-36bc-46bf-8ab6-f7d7d47e37d9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_StockOptionsAndRestrictedStockUnitsOutstandingMember_8ce56b26-9eab-4cc3-914f-c0de2f90b079" xlink:href="ftnt-20211231.xsd#ftnt_StockOptionsAndRestrictedStockUnitsOutstandingMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_6dda3b6f-3fa5-495a-884b-0bca3fb8d289" xlink:to="loc_ftnt_StockOptionsAndRestrictedStockUnitsOutstandingMember_8ce56b26-9eab-4cc3-914f-c0de2f90b079" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/EquityAllocationofStockBasedCompensationExpenseDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#EquityAllocationofStockBasedCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/EquityAllocationofStockBasedCompensationExpenseDetails" xlink:type="extended" id="i37bdb17e7060472bbe1661479fccbb20_EquityAllocationofStockBasedCompensationExpenseDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_76d50dcd-e2f9-4478-8a64-ea07f1f37b43" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_42b4ebef-2868-45e0-a449-ccc7fc5a543e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_76d50dcd-e2f9-4478-8a64-ea07f1f37b43" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_42b4ebef-2868-45e0-a449-ccc7fc5a543e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions_11648889-ca67-4915-b093-087ca159eba0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_76d50dcd-e2f9-4478-8a64-ea07f1f37b43" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions_11648889-ca67-4915-b093-087ca159eba0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_0720faf5-e775-4d1b-a637-8dc64ed3cb42" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_76d50dcd-e2f9-4478-8a64-ea07f1f37b43" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_0720faf5-e775-4d1b-a637-8dc64ed3cb42" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_9fb3aae5-fca5-4278-8815-675739c75c48" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_0720faf5-e775-4d1b-a637-8dc64ed3cb42" xlink:to="loc_us-gaap_AwardTypeAxis_9fb3aae5-fca5-4278-8815-675739c75c48" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_9fb3aae5-fca5-4278-8815-675739c75c48_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_9fb3aae5-fca5-4278-8815-675739c75c48" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_9fb3aae5-fca5-4278-8815-675739c75c48_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f7aae9e5-1573-40f3-9c8c-97c9bb044294" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_9fb3aae5-fca5-4278-8815-675739c75c48" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f7aae9e5-1573-40f3-9c8c-97c9bb044294" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_2f532594-7818-4fc1-8d53-cb54754f02e5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f7aae9e5-1573-40f3-9c8c-97c9bb044294" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_2f532594-7818-4fc1-8d53-cb54754f02e5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_ccf52d15-b3ee-4b2a-bc51-6ae2ec73f9ed" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f7aae9e5-1573-40f3-9c8c-97c9bb044294" xlink:to="loc_us-gaap_EmployeeStockOptionMember_ccf52d15-b3ee-4b2a-bc51-6ae2ec73f9ed" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockMember_08d2b78d-0ff9-4ae6-9095-baa67552d3be" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_EmployeeStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_f7aae9e5-1573-40f3-9c8c-97c9bb044294" xlink:to="loc_us-gaap_EmployeeStockMember_08d2b78d-0ff9-4ae6-9095-baa67552d3be" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_baf9b5ef-9471-4240-b1d1-802af67820a3" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_0720faf5-e775-4d1b-a637-8dc64ed3cb42" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_baf9b5ef-9471-4240-b1d1-802af67820a3" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_baf9b5ef-9471-4240-b1d1-802af67820a3_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_baf9b5ef-9471-4240-b1d1-802af67820a3" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_baf9b5ef-9471-4240-b1d1-802af67820a3_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_d2921eec-cc91-42bf-a651-689db01fc04e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_baf9b5ef-9471-4240-b1d1-802af67820a3" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_d2921eec-cc91-42bf-a651-689db01fc04e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_CostOfGoodsSoldMember_d5a71414-b3a8-4612-8665-fbd107fcd955" xlink:href="ftnt-20211231.xsd#ftnt_CostOfGoodsSoldMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_d2921eec-cc91-42bf-a651-689db01fc04e" xlink:to="loc_ftnt_CostOfGoodsSoldMember_d5a71414-b3a8-4612-8665-fbd107fcd955" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_CostOfServicesMember_5593d22a-1647-4b61-bccd-692dee8f1435" xlink:href="ftnt-20211231.xsd#ftnt_CostOfServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_d2921eec-cc91-42bf-a651-689db01fc04e" xlink:to="loc_ftnt_CostOfServicesMember_5593d22a-1647-4b61-bccd-692dee8f1435" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_cd58a8a6-c0dd-4c36-b003-113c804122cd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ResearchAndDevelopmentExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_d2921eec-cc91-42bf-a651-689db01fc04e" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_cd58a8a6-c0dd-4c36-b003-113c804122cd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingAndMarketingExpenseMember_7a56454c-2f57-4f1e-aeb3-26c378105f68" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SellingAndMarketingExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_d2921eec-cc91-42bf-a651-689db01fc04e" xlink:to="loc_us-gaap_SellingAndMarketingExpenseMember_7a56454c-2f57-4f1e-aeb3-26c378105f68" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpenseMember_081f8494-ac47-4534-a6b1-6dffe04e53ee" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_GeneralAndAdministrativeExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_d2921eec-cc91-42bf-a651-689db01fc04e" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpenseMember_081f8494-ac47-4534-a6b1-6dffe04e53ee" xlink:type="arc" order="4"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/EquityShareRepurchaseProgramDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#EquityShareRepurchaseProgramDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/EquityShareRepurchaseProgramDetails" xlink:type="extended" id="id3995a94f858444eb70f582c71c80284_EquityShareRepurchaseProgramDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e54a2cab-3553-4e84-aa2f-6e50c8d65dac" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchaseProgramAuthorizedAmount1_7344aea8-94c8-4d11-8817-22949eb7ccb6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockRepurchaseProgramAuthorizedAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e54a2cab-3553-4e84-aa2f-6e50c8d65dac" xlink:to="loc_us-gaap_StockRepurchaseProgramAuthorizedAmount1_7344aea8-94c8-4d11-8817-22949eb7ccb6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_StockRepurchaseProgramAuthorizedAmountApprovedIncrease_caa6ee21-58f3-47c3-84d1-718877973687" xlink:href="ftnt-20211231.xsd#ftnt_StockRepurchaseProgramAuthorizedAmountApprovedIncrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e54a2cab-3553-4e84-aa2f-6e50c8d65dac" xlink:to="loc_ftnt_StockRepurchaseProgramAuthorizedAmountApprovedIncrease_caa6ee21-58f3-47c3-84d1-718877973687" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_c978a8cc-cb8b-4ef3-912e-6e700c0b1a19" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e54a2cab-3553-4e84-aa2f-6e50c8d65dac" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_c978a8cc-cb8b-4ef3-912e-6e700c0b1a19" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_64bd40f8-1c0b-4144-b6b7-029ee535bd46" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e54a2cab-3553-4e84-aa2f-6e50c8d65dac" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_64bd40f8-1c0b-4144-b6b7-029ee535bd46" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_3cbdc8a4-7d93-495a-b66f-f369d84e179f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e54a2cab-3553-4e84-aa2f-6e50c8d65dac" xlink:to="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_3cbdc8a4-7d93-495a-b66f-f369d84e179f" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_c6c78c25-30da-4481-8d0d-e6fa2f5f5fe9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_e54a2cab-3553-4e84-aa2f-6e50c8d65dac" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_c6c78c25-30da-4481-8d0d-e6fa2f5f5fe9" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareRepurchaseProgramAxis_386fdc41-477f-49ed-b0cb-ebdbb1747447" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareRepurchaseProgramAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_c6c78c25-30da-4481-8d0d-e6fa2f5f5fe9" xlink:to="loc_us-gaap_ShareRepurchaseProgramAxis_386fdc41-477f-49ed-b0cb-ebdbb1747447" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_386fdc41-477f-49ed-b0cb-ebdbb1747447_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_386fdc41-477f-49ed-b0cb-ebdbb1747447" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_386fdc41-477f-49ed-b0cb-ebdbb1747447_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_082a6966-545c-4080-916c-fae632f1bec6" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_386fdc41-477f-49ed-b0cb-ebdbb1747447" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_082a6966-545c-4080-916c-fae632f1bec6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ShareRepurchaseProgramMember_475e57f2-72b3-420d-afa2-7b113bf0b127" xlink:href="ftnt-20211231.xsd#ftnt_ShareRepurchaseProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareRepurchaseProgramDomain_082a6966-545c-4080-916c-fae632f1bec6" xlink:to="loc_ftnt_ShareRepurchaseProgramMember_475e57f2-72b3-420d-afa2-7b113bf0b127" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_3b3987c0-b40d-48c5-89bd-c46de2e8652f" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_c6c78c25-30da-4481-8d0d-e6fa2f5f5fe9" xlink:to="loc_srt_RangeAxis_3b3987c0-b40d-48c5-89bd-c46de2e8652f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_3b3987c0-b40d-48c5-89bd-c46de2e8652f_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_3b3987c0-b40d-48c5-89bd-c46de2e8652f" xlink:to="loc_srt_RangeMember_3b3987c0-b40d-48c5-89bd-c46de2e8652f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_2b88a480-919e-41c1-a654-7d9cea29eda0" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_3b3987c0-b40d-48c5-89bd-c46de2e8652f" xlink:to="loc_srt_RangeMember_2b88a480-919e-41c1-a654-7d9cea29eda0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_caeeef3d-98a5-4b22-9996-08d335c794d0" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_2b88a480-919e-41c1-a654-7d9cea29eda0" xlink:to="loc_srt_MaximumMember_caeeef3d-98a5-4b22-9996-08d335c794d0" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/IncomeTaxesNarrativeandDeferredTaxAssetsDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#IncomeTaxesNarrativeandDeferredTaxAssetsDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/IncomeTaxesNarrativeandDeferredTaxAssetsDetails" xlink:type="extended" id="i539790d32cb447e291293085b67740f0_IncomeTaxesNarrativeandDeferredTaxAssetsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_DeferredTaxesLineItems_c4f9a67d-4b0d-4cd1-9b1d-36b0a4d7316b" xlink:href="ftnt-20211231.xsd#ftnt_DeferredTaxesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComponentsOfDeferredTaxAssetsAbstract_787f7b97-2cc0-4216-a2a3-d28b226667a8" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ComponentsOfDeferredTaxAssetsAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DeferredTaxesLineItems_c4f9a67d-4b0d-4cd1-9b1d-36b0a4d7316b" xlink:to="loc_us-gaap_ComponentsOfDeferredTaxAssetsAbstract_787f7b97-2cc0-4216-a2a3-d28b226667a8" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsGeneralBusiness_bf2908ec-6291-4b96-954f-e0a6239cfbef" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsGeneralBusiness"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComponentsOfDeferredTaxAssetsAbstract_787f7b97-2cc0-4216-a2a3-d28b226667a8" xlink:to="loc_us-gaap_DeferredTaxAssetsTaxCreditCarryforwardsGeneralBusiness_bf2908ec-6291-4b96-954f-e0a6239cfbef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsDeferredIncome_922a4cd8-8d17-46ab-b2a6-4d5a106b3673" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxAssetsDeferredIncome"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComponentsOfDeferredTaxAssetsAbstract_787f7b97-2cc0-4216-a2a3-d28b226667a8" xlink:to="loc_us-gaap_DeferredTaxAssetsDeferredIncome_922a4cd8-8d17-46ab-b2a6-4d5a106b3673" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccruals_bcd2f705-4ed5-4c90-b62a-0ede89e1e1d4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccruals"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComponentsOfDeferredTaxAssetsAbstract_787f7b97-2cc0-4216-a2a3-d28b226667a8" xlink:to="loc_us-gaap_DeferredTaxAssetsTaxDeferredExpenseReservesAndAccruals_bcd2f705-4ed5-4c90-b62a-0ede89e1e1d4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwards_140b69c9-8f75-4f8d-8cda-1bcb9b1cdfa9" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxAssetsOperatingLossCarryforwards"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComponentsOfDeferredTaxAssetsAbstract_787f7b97-2cc0-4216-a2a3-d28b226667a8" xlink:to="loc_us-gaap_DeferredTaxAssetsOperatingLossCarryforwards_140b69c9-8f75-4f8d-8cda-1bcb9b1cdfa9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost_a260a016-7a06-4b2d-a383-5b7e5081088d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComponentsOfDeferredTaxAssetsAbstract_787f7b97-2cc0-4216-a2a3-d28b226667a8" xlink:to="loc_us-gaap_DeferredTaxAssetsTaxDeferredExpenseCompensationAndBenefitsShareBasedCompensationCost_a260a016-7a06-4b2d-a383-5b7e5081088d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsPropertyPlantAndEquipment_a41f1beb-afa6-4e14-ae63-97b2c6ef6295" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxAssetsPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComponentsOfDeferredTaxAssetsAbstract_787f7b97-2cc0-4216-a2a3-d28b226667a8" xlink:to="loc_us-gaap_DeferredTaxAssetsPropertyPlantAndEquipment_a41f1beb-afa6-4e14-ae63-97b2c6ef6295" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_DeferredTaxAssetsResearchAndDevelopmentCapitalized_04cd97c4-f518-45cf-88dd-332aca402567" xlink:href="ftnt-20211231.xsd#ftnt_DeferredTaxAssetsResearchAndDevelopmentCapitalized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComponentsOfDeferredTaxAssetsAbstract_787f7b97-2cc0-4216-a2a3-d28b226667a8" xlink:to="loc_ftnt_DeferredTaxAssetsResearchAndDevelopmentCapitalized_04cd97c4-f518-45cf-88dd-332aca402567" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_DeferredTaxAssetsOperatingLeaseLiabilities_2f1d1660-5285-4a1b-ba70-ebc9bfcc25ec" xlink:href="ftnt-20211231.xsd#ftnt_DeferredTaxAssetsOperatingLeaseLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComponentsOfDeferredTaxAssetsAbstract_787f7b97-2cc0-4216-a2a3-d28b226667a8" xlink:to="loc_ftnt_DeferredTaxAssetsOperatingLeaseLiabilities_2f1d1660-5285-4a1b-ba70-ebc9bfcc25ec" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsGross_19688616-10e6-41f5-beec-6e6957e31c7f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComponentsOfDeferredTaxAssetsAbstract_787f7b97-2cc0-4216-a2a3-d28b226667a8" xlink:to="loc_us-gaap_DeferredTaxAssetsGross_19688616-10e6-41f5-beec-6e6957e31c7f" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsValuationAllowance_15e38cbf-89ca-45cb-872f-b6ba68d52824" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxAssetsValuationAllowance"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComponentsOfDeferredTaxAssetsAbstract_787f7b97-2cc0-4216-a2a3-d28b226667a8" xlink:to="loc_us-gaap_DeferredTaxAssetsValuationAllowance_15e38cbf-89ca-45cb-872f-b6ba68d52824" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsNet_c1e82278-1d81-4b92-b7ed-76de671973ff" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ComponentsOfDeferredTaxAssetsAbstract_787f7b97-2cc0-4216-a2a3-d28b226667a8" xlink:to="loc_us-gaap_DeferredTaxAssetsNet_c1e82278-1d81-4b92-b7ed-76de671973ff" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxLiabilitiesAbstract_e38050bc-fd2c-4564-bf7a-c30dae461a2b" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxLiabilitiesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DeferredTaxesLineItems_c4f9a67d-4b0d-4cd1-9b1d-36b0a4d7316b" xlink:to="loc_us-gaap_DeferredTaxLiabilitiesAbstract_e38050bc-fd2c-4564-bf7a-c30dae461a2b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxLiabilitiesDeferredExpense_98868b61-7dca-4ade-b005-53dd30695077" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxLiabilitiesDeferredExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredTaxLiabilitiesAbstract_e38050bc-fd2c-4564-bf7a-c30dae461a2b" xlink:to="loc_us-gaap_DeferredTaxLiabilitiesDeferredExpense_98868b61-7dca-4ade-b005-53dd30695077" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxLiabilitiesLeasingArrangements_3b9b09d7-2208-444d-bfa1-3c6c993507b1" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxLiabilitiesLeasingArrangements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredTaxLiabilitiesAbstract_e38050bc-fd2c-4564-bf7a-c30dae461a2b" xlink:to="loc_us-gaap_DeferredTaxLiabilitiesLeasingArrangements_3b9b09d7-2208-444d-bfa1-3c6c993507b1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxLiabilitiesGoodwillAndIntangibleAssetsIntangibleAssets_1f53209a-29b2-4e00-b458-42bda8be6e25" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxLiabilitiesGoodwillAndIntangibleAssetsIntangibleAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredTaxLiabilitiesAbstract_e38050bc-fd2c-4564-bf7a-c30dae461a2b" xlink:to="loc_us-gaap_DeferredTaxLiabilitiesGoodwillAndIntangibleAssetsIntangibleAssets_1f53209a-29b2-4e00-b458-42bda8be6e25" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxLiabilities_a97cec49-47da-49a7-a534-2ae9831c9944" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredIncomeTaxLiabilities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DeferredTaxLiabilitiesAbstract_e38050bc-fd2c-4564-bf7a-c30dae461a2b" xlink:to="loc_us-gaap_DeferredIncomeTaxLiabilities_a97cec49-47da-49a7-a534-2ae9831c9944" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsLiabilitiesNet_72a0bb25-ef76-4cd6-9a65-aa29da173507" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredTaxAssetsLiabilitiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DeferredTaxesLineItems_c4f9a67d-4b0d-4cd1-9b1d-36b0a4d7316b" xlink:to="loc_us-gaap_DeferredTaxAssetsLiabilitiesNet_72a0bb25-ef76-4cd6-9a65-aa29da173507" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxAssetsNet_98d4d3fb-feda-417e-adde-f9ccb16e428e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredIncomeTaxAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DeferredTaxesLineItems_c4f9a67d-4b0d-4cd1-9b1d-36b0a4d7316b" xlink:to="loc_us-gaap_DeferredIncomeTaxAssetsNet_98d4d3fb-feda-417e-adde-f9ccb16e428e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_28781e1a-67b3-4b46-b211-7cf73f8073cd" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DeferredIncomeTaxLiabilitiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DeferredTaxesLineItems_c4f9a67d-4b0d-4cd1-9b1d-36b0a4d7316b" xlink:to="loc_us-gaap_DeferredIncomeTaxLiabilitiesNet_28781e1a-67b3-4b46-b211-7cf73f8073cd" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLossCarryforwards_1f5a7e9c-8d18-4af5-b8f7-996e9216c30a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OperatingLossCarryforwards"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DeferredTaxesLineItems_c4f9a67d-4b0d-4cd1-9b1d-36b0a4d7316b" xlink:to="loc_us-gaap_OperatingLossCarryforwards_1f5a7e9c-8d18-4af5-b8f7-996e9216c30a" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TaxCreditCarryforwardAmount_96885dbd-65b6-4b38-a007-733a9d3060ab" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TaxCreditCarryforwardAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DeferredTaxesLineItems_c4f9a67d-4b0d-4cd1-9b1d-36b0a4d7316b" xlink:to="loc_us-gaap_TaxCreditCarryforwardAmount_96885dbd-65b6-4b38-a007-733a9d3060ab" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ScheduleOfDeferredTaxesTable_79105d01-47db-49cd-a79f-d28e92c12398" xlink:href="ftnt-20211231.xsd#ftnt_ScheduleOfDeferredTaxesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_ftnt_DeferredTaxesLineItems_c4f9a67d-4b0d-4cd1-9b1d-36b0a4d7316b" xlink:to="loc_ftnt_ScheduleOfDeferredTaxesTable_79105d01-47db-49cd-a79f-d28e92c12398" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_808fe7c3-6857-435d-9910-fafc5421a61e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ftnt_ScheduleOfDeferredTaxesTable_79105d01-47db-49cd-a79f-d28e92c12398" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_808fe7c3-6857-435d-9910-fafc5421a61e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_808fe7c3-6857-435d-9910-fafc5421a61e_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_808fe7c3-6857-435d-9910-fafc5421a61e" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_808fe7c3-6857-435d-9910-fafc5421a61e_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_67300378-63a1-49e6-b282-b7c4803b1b49" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_808fe7c3-6857-435d-9910-fafc5421a61e" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_67300378-63a1-49e6-b282-b7c4803b1b49" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_AccruedIncomeTaxesNoncurrentMember_35128793-c9fb-4408-928e-b057a9439a97" xlink:href="ftnt-20211231.xsd#ftnt_AccruedIncomeTaxesNoncurrentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_67300378-63a1-49e6-b282-b7c4803b1b49" xlink:to="loc_ftnt_AccruedIncomeTaxesNoncurrentMember_35128793-c9fb-4408-928e-b057a9439a97" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityAxis_cddcee48-b202-4376-ae85-e6665f328114" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxAuthorityAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ftnt_ScheduleOfDeferredTaxesTable_79105d01-47db-49cd-a79f-d28e92c12398" xlink:to="loc_us-gaap_IncomeTaxAuthorityAxis_cddcee48-b202-4376-ae85-e6665f328114" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_cddcee48-b202-4376-ae85-e6665f328114_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_cddcee48-b202-4376-ae85-e6665f328114" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_cddcee48-b202-4376-ae85-e6665f328114_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxAuthorityDomain_087f4722-a07b-4d95-a886-407e5e674bc5" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxAuthorityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeTaxAuthorityAxis_cddcee48-b202-4376-ae85-e6665f328114" xlink:to="loc_us-gaap_IncomeTaxAuthorityDomain_087f4722-a07b-4d95-a886-407e5e674bc5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DomesticCountryMember_dc746f8c-a6ef-4a4b-a15a-4b5fd592beef" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DomesticCountryMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_087f4722-a07b-4d95-a886-407e5e674bc5" xlink:to="loc_us-gaap_DomesticCountryMember_dc746f8c-a6ef-4a4b-a15a-4b5fd592beef" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StateAndLocalJurisdictionMember_52e063c8-ca24-40b8-ae6d-ff80441ba77a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StateAndLocalJurisdictionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxAuthorityDomain_087f4722-a07b-4d95-a886-407e5e674bc5" xlink:to="loc_us-gaap_StateAndLocalJurisdictionMember_52e063c8-ca24-40b8-ae6d-ff80441ba77a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_StateJurisdictionCaliforniaMember_71cc96f0-5acc-4f1c-aae5-8af5e0e05349" xlink:href="ftnt-20211231.xsd#ftnt_StateJurisdictionCaliforniaMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StateAndLocalJurisdictionMember_52e063c8-ca24-40b8-ae6d-ff80441ba77a" xlink:to="loc_ftnt_StateJurisdictionCaliforniaMember_71cc96f0-5acc-4f1c-aae5-8af5e0e05349" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/IncomeTaxesUnrecognizedTaxBenefitsDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#IncomeTaxesUnrecognizedTaxBenefitsDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/IncomeTaxesUnrecognizedTaxBenefitsDetails" xlink:type="extended" id="i789f548a02754c438324387d1bd80010_IncomeTaxesUnrecognizedTaxBenefitsDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxContingencyLineItems_e956ff7d-8503-4a21-a8eb-9c224d70c2c7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxContingencyLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate_b437d702-0051-4e2d-9de4-a1a3cf3bedff" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_e956ff7d-8503-4a21-a8eb-9c224d70c2c7" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate_b437d702-0051-4e2d-9de4-a1a3cf3bedff" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued_49b856e6-cd2f-4d87-b4a9-e44e2a654eb0" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_e956ff7d-8503-4a21-a8eb-9c224d70c2c7" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued_49b856e6-cd2f-4d87-b4a9-e44e2a654eb0" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ReconciliationOfUnrecognizedTaxBenefitsExcludingAmountsPertainingToExaminedTaxReturnsRollForward_f2e4ec1f-e92e-4121-bf1a-82de32a8fd1c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ReconciliationOfUnrecognizedTaxBenefitsExcludingAmountsPertainingToExaminedTaxReturnsRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_e956ff7d-8503-4a21-a8eb-9c224d70c2c7" xlink:to="loc_us-gaap_ReconciliationOfUnrecognizedTaxBenefitsExcludingAmountsPertainingToExaminedTaxReturnsRollForward_f2e4ec1f-e92e-4121-bf1a-82de32a8fd1c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefits_7b6e62c6-4f90-4bbc-af9e-d5dce65093da" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_UnrecognizedTaxBenefits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReconciliationOfUnrecognizedTaxBenefitsExcludingAmountsPertainingToExaminedTaxReturnsRollForward_f2e4ec1f-e92e-4121-bf1a-82de32a8fd1c" xlink:to="loc_us-gaap_UnrecognizedTaxBenefits_7b6e62c6-4f90-4bbc-af9e-d5dce65093da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions_3e24657b-c8cc-4af7-9ec9-9c70af59e187" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReconciliationOfUnrecognizedTaxBenefitsExcludingAmountsPertainingToExaminedTaxReturnsRollForward_f2e4ec1f-e92e-4121-bf1a-82de32a8fd1c" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromCurrentPeriodTaxPositions_3e24657b-c8cc-4af7-9ec9-9c70af59e187" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions_1684135f-2dbb-4ff1-b10f-69af0bc95a83" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReconciliationOfUnrecognizedTaxBenefitsExcludingAmountsPertainingToExaminedTaxReturnsRollForward_f2e4ec1f-e92e-4121-bf1a-82de32a8fd1c" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromCurrentPeriodTaxPositions_1684135f-2dbb-4ff1-b10f-69af0bc95a83" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions_d8403f4b-a9bb-4d40-8936-f6d65e3ccc51" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReconciliationOfUnrecognizedTaxBenefitsExcludingAmountsPertainingToExaminedTaxReturnsRollForward_f2e4ec1f-e92e-4121-bf1a-82de32a8fd1c" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsIncreasesResultingFromPriorPeriodTaxPositions_d8403f4b-a9bb-4d40-8936-f6d65e3ccc51" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions_e6c2a919-dd79-4d1a-b3f0-3ecd1bd11b8f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReconciliationOfUnrecognizedTaxBenefitsExcludingAmountsPertainingToExaminedTaxReturnsRollForward_f2e4ec1f-e92e-4121-bf1a-82de32a8fd1c" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromPriorPeriodTaxPositions_e6c2a919-dd79-4d1a-b3f0-3ecd1bd11b8f" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities_5647dd95-d8ea-4514-8a7a-e0d57a0cf4ee" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReconciliationOfUnrecognizedTaxBenefitsExcludingAmountsPertainingToExaminedTaxReturnsRollForward_f2e4ec1f-e92e-4121-bf1a-82de32a8fd1c" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsDecreasesResultingFromSettlementsWithTaxingAuthorities_5647dd95-d8ea-4514-8a7a-e0d57a0cf4ee" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations_9107a174-b98b-4935-b528-0b1ca05eef35" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReconciliationOfUnrecognizedTaxBenefitsExcludingAmountsPertainingToExaminedTaxReturnsRollForward_f2e4ec1f-e92e-4121-bf1a-82de32a8fd1c" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsReductionsResultingFromLapseOfApplicableStatuteOfLimitations_9107a174-b98b-4935-b528-0b1ca05eef35" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefits_d8276054-4595-42e0-8d29-d90bc288f8ad" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_UnrecognizedTaxBenefits"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsPeriodIncreaseDecrease_f69dab29-ca65-4620-8302-906f079344e7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsPeriodIncreaseDecrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_e956ff7d-8503-4a21-a8eb-9c224d70c2c7" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsPeriodIncreaseDecrease_f69dab29-ca65-4620-8302-906f079344e7" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilityForUncertainTaxPositionsNoncurrent_7b818059-c994-4586-a5d0-0de63ab71568" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LiabilityForUncertainTaxPositionsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_e956ff7d-8503-4a21-a8eb-9c224d70c2c7" xlink:to="loc_us-gaap_LiabilityForUncertainTaxPositionsNoncurrent_7b818059-c994-4586-a5d0-0de63ab71568" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DecreaseInUnrecognizedTaxBenefitsIsReasonablyPossible_9524da30-eeec-412f-9beb-bcace3976f2c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_DecreaseInUnrecognizedTaxBenefitsIsReasonablyPossible"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_e956ff7d-8503-4a21-a8eb-9c224d70c2c7" xlink:to="loc_us-gaap_DecreaseInUnrecognizedTaxBenefitsIsReasonablyPossible_9524da30-eeec-412f-9beb-bcace3976f2c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxContingencyTable_ad53f1aa-201f-438e-b5c3-7c718210a174" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_IncomeTaxContingencyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_e956ff7d-8503-4a21-a8eb-9c224d70c2c7" xlink:to="loc_us-gaap_IncomeTaxContingencyTable_ad53f1aa-201f-438e-b5c3-7c718210a174" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_efc397a3-df81-4065-9ebc-d22728b5e219" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeTaxContingencyTable_ad53f1aa-201f-438e-b5c3-7c718210a174" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_efc397a3-df81-4065-9ebc-d22728b5e219" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_efc397a3-df81-4065-9ebc-d22728b5e219_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_efc397a3-df81-4065-9ebc-d22728b5e219" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_efc397a3-df81-4065-9ebc-d22728b5e219_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_9f46654b-4146-4949-816d-58d01ee17312" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_efc397a3-df81-4065-9ebc-d22728b5e219" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_9f46654b-4146-4949-816d-58d01ee17312" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_IncomeTaxLiabilitiesNoncurrentMember_7a53a7ea-4b20-46d5-8044-2eec7ca8f700" xlink:href="ftnt-20211231.xsd#ftnt_IncomeTaxLiabilitiesNoncurrentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_9f46654b-4146-4949-816d-58d01ee17312" xlink:to="loc_ftnt_IncomeTaxLiabilitiesNoncurrentMember_7a53a7ea-4b20-46d5-8044-2eec7ca8f700" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/SegmentInformationDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#SegmentInformationDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/SegmentInformationDetails" xlink:type="extended" id="idc56d659a29d46d2b5e73169ac80e27c_SegmentInformationDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_74f57193-c93e-4ceb-97ee-798ad4d26fb4" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_NumberOfBusinessActivities_e92b9c17-612d-411a-8c8b-90108e83b75c" xlink:href="ftnt-20211231.xsd#ftnt_NumberOfBusinessActivities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_74f57193-c93e-4ceb-97ee-798ad4d26fb4" xlink:to="loc_ftnt_NumberOfBusinessActivities_e92b9c17-612d-411a-8c8b-90108e83b75c" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_NumberOfSegmentManagersResponsibleForOperations_188a3c64-14ea-4655-a58b-dcf00d3d3d55" xlink:href="ftnt-20211231.xsd#ftnt_NumberOfSegmentManagersResponsibleForOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_74f57193-c93e-4ceb-97ee-798ad4d26fb4" xlink:to="loc_ftnt_NumberOfSegmentManagersResponsibleForOperations_188a3c64-14ea-4655-a58b-dcf00d3d3d55" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfOperatingSegments_d5a808be-3134-4d52-89b3-57e1f445cd72" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NumberOfOperatingSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_74f57193-c93e-4ceb-97ee-798ad4d26fb4" xlink:to="loc_us-gaap_NumberOfOperatingSegments_d5a808be-3134-4d52-89b3-57e1f445cd72" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NumberOfReportableSegments_a9d4b676-37df-451f-8712-9d889e67338c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_NumberOfReportableSegments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_74f57193-c93e-4ceb-97ee-798ad4d26fb4" xlink:to="loc_us-gaap_NumberOfReportableSegments_a9d4b676-37df-451f-8712-9d889e67338c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_ccdb9820-2ceb-4a97-b190-453921eedebf" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_74f57193-c93e-4ceb-97ee-798ad4d26fb4" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_ccdb9820-2ceb-4a97-b190-453921eedebf" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_d31d3a52-4678-4434-80b2-246c19bd106d" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_74f57193-c93e-4ceb-97ee-798ad4d26fb4" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_d31d3a52-4678-4434-80b2-246c19bd106d" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_2eab1cdf-c7ca-4cac-8148-cc1cd7733e3a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_74f57193-c93e-4ceb-97ee-798ad4d26fb4" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_2eab1cdf-c7ca-4cac-8148-cc1cd7733e3a" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_0a3444e0-780e-4bcd-9fa4-fd344e3b0739" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_2eab1cdf-c7ca-4cac-8148-cc1cd7733e3a" xlink:to="loc_srt_StatementGeographicalAxis_0a3444e0-780e-4bcd-9fa4-fd344e3b0739" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_0a3444e0-780e-4bcd-9fa4-fd344e3b0739_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_0a3444e0-780e-4bcd-9fa4-fd344e3b0739" xlink:to="loc_srt_SegmentGeographicalDomain_0a3444e0-780e-4bcd-9fa4-fd344e3b0739_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_16572573-5370-4545-958e-6e8788dd861e" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_0a3444e0-780e-4bcd-9fa4-fd344e3b0739" xlink:to="loc_srt_SegmentGeographicalDomain_16572573-5370-4545-958e-6e8788dd861e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_AmericasMember_17565fa8-8634-4208-bf10-b409651b16c1" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_AmericasMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_16572573-5370-4545-958e-6e8788dd861e" xlink:to="loc_srt_AmericasMember_17565fa8-8634-4208-bf10-b409651b16c1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_9f9128ee-73e6-4cd5-90d8-b6a8af9a4e13" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_AmericasMember_17565fa8-8634-4208-bf10-b409651b16c1" xlink:to="loc_country_US_9f9128ee-73e6-4cd5-90d8-b6a8af9a4e13" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_CA_c6c6b2f7-a220-49bf-a97e-04b180d9781e" xlink:href="https://xbrl.sec.gov/country/2021/country-2021.xsd#country_CA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_AmericasMember_17565fa8-8634-4208-bf10-b409651b16c1" xlink:to="loc_country_CA_c6c6b2f7-a220-49bf-a97e-04b180d9781e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LatinAmericaMember_0be07b13-561b-4d90-84b9-32b289b07908" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_LatinAmericaMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_AmericasMember_17565fa8-8634-4208-bf10-b409651b16c1" xlink:to="loc_srt_LatinAmericaMember_0be07b13-561b-4d90-84b9-32b289b07908" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_OtherAmericasMember_c35f1633-bfea-4ea3-8efb-996d4dbb2c15" xlink:href="ftnt-20211231.xsd#ftnt_OtherAmericasMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_AmericasMember_17565fa8-8634-4208-bf10-b409651b16c1" xlink:to="loc_ftnt_OtherAmericasMember_c35f1633-bfea-4ea3-8efb-996d4dbb2c15" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_EuropeMiddleEastAndAfricaMember_402db03c-2840-4ec7-8cf1-2c0bdb0e624e" xlink:href="ftnt-20211231.xsd#ftnt_EuropeMiddleEastAndAfricaMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_16572573-5370-4545-958e-6e8788dd861e" xlink:to="loc_ftnt_EuropeMiddleEastAndAfricaMember_402db03c-2840-4ec7-8cf1-2c0bdb0e624e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_AsiaPacificAndJapanMember_a66734a6-9321-4c51-81b0-fec523967fbc" xlink:href="ftnt-20211231.xsd#ftnt_AsiaPacificAndJapanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_16572573-5370-4545-958e-6e8788dd861e" xlink:to="loc_ftnt_AsiaPacificAndJapanMember_a66734a6-9321-4c51-81b0-fec523967fbc" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/SubsequentEventDetails" xlink:type="simple" xlink:href="ftnt-20211231.xsd#SubsequentEventDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/SubsequentEventDetails" xlink:type="extended" id="i808fd99d1e2a47dbb3459766c0ff9355_SubsequentEventDetails">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_37eef265-2a27-42ad-b4b9-a25ed3438884" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_PropertyPlantAndEquipmentAdditionsAreaOfPropertyPurchased_8c51c128-6b93-4c95-b446-6bfca502d96e" xlink:href="ftnt-20211231.xsd#ftnt_PropertyPlantAndEquipmentAdditionsAreaOfPropertyPurchased"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_37eef265-2a27-42ad-b4b9-a25ed3438884" xlink:to="loc_ftnt_PropertyPlantAndEquipmentAdditionsAreaOfPropertyPurchased_8c51c128-6b93-4c95-b446-6bfca502d96e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_d120c0ee-1f1a-4a66-84d3-3616d0cadee7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_37eef265-2a27-42ad-b4b9-a25ed3438884" xlink:to="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_d120c0ee-1f1a-4a66-84d3-3616d0cadee7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedDuringPeriodShares_595b2f48-f35c-46d0-9c61-c1079f83880e" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockRepurchasedDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_37eef265-2a27-42ad-b4b9-a25ed3438884" xlink:to="loc_us-gaap_StockRepurchasedDuringPeriodShares_595b2f48-f35c-46d0-9c61-c1079f83880e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedDuringPeriodValue_908888d2-c349-49b6-ba3c-e460303b1b67" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StockRepurchasedDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_37eef265-2a27-42ad-b4b9-a25ed3438884" xlink:to="loc_us-gaap_StockRepurchasedDuringPeriodValue_908888d2-c349-49b6-ba3c-e460303b1b67" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_68a41a17-d667-4007-8ede-81efa52248a7" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_TreasuryStockAcquiredAverageCostPerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_37eef265-2a27-42ad-b4b9-a25ed3438884" xlink:to="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_68a41a17-d667-4007-8ede-81efa52248a7" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_e4ff30b5-dc88-43cd-b4f3-42b328dd4f5c" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_37eef265-2a27-42ad-b4b9-a25ed3438884" xlink:to="loc_us-gaap_SubsequentEventTable_e4ff30b5-dc88-43cd-b4f3-42b328dd4f5c" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_cd5ac5f2-813a-485e-971b-56baa20e0887" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_e4ff30b5-dc88-43cd-b4f3-42b328dd4f5c" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_cd5ac5f2-813a-485e-971b-56baa20e0887" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_cd5ac5f2-813a-485e-971b-56baa20e0887_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_cd5ac5f2-813a-485e-971b-56baa20e0887" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_cd5ac5f2-813a-485e-971b-56baa20e0887_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_90790c22-cee6-47e2-b445-77a573d9fa8a" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_cd5ac5f2-813a-485e-971b-56baa20e0887" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_90790c22-cee6-47e2-b445-77a573d9fa8a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_39832f50-be1f-4cd9-b420-1c203455fc85" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_90790c22-cee6-47e2-b445-77a573d9fa8a" xlink:to="loc_us-gaap_SubsequentEventMember_39832f50-be1f-4cd9-b420-1c203455fc85" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_db6a12c0-d55e-4bc2-a67c-b3c443dfa2fc" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_e4ff30b5-dc88-43cd-b4f3-42b328dd4f5c" xlink:to="loc_srt_StatementGeographicalAxis_db6a12c0-d55e-4bc2-a67c-b3c443dfa2fc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_db6a12c0-d55e-4bc2-a67c-b3c443dfa2fc_default" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_db6a12c0-d55e-4bc2-a67c-b3c443dfa2fc" xlink:to="loc_srt_SegmentGeographicalDomain_db6a12c0-d55e-4bc2-a67c-b3c443dfa2fc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_11fa01c2-13e9-4bae-9f0f-2659be068eee" xlink:href="https://xbrl.fasb.org/srt/2021/elts/srt-2021-01-31.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_db6a12c0-d55e-4bc2-a67c-b3c443dfa2fc" xlink:to="loc_srt_SegmentGeographicalDomain_11fa01c2-13e9-4bae-9f0f-2659be068eee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ChicagoMember_f354fa7d-e243-4212-85b9-f2ae0c3b9d9a" xlink:href="ftnt-20211231.xsd#ftnt_ChicagoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_11fa01c2-13e9-4bae-9f0f-2659be068eee" xlink:to="loc_ftnt_ChicagoMember_f354fa7d-e243-4212-85b9-f2ae0c3b9d9a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementClassOfStockAxis_204410ba-923a-42c3-91d1-fb3eb043a29f" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_StatementClassOfStockAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_e4ff30b5-dc88-43cd-b4f3-42b328dd4f5c" xlink:to="loc_us-gaap_StatementClassOfStockAxis_204410ba-923a-42c3-91d1-fb3eb043a29f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_204410ba-923a-42c3-91d1-fb3eb043a29f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementClassOfStockAxis_204410ba-923a-42c3-91d1-fb3eb043a29f" xlink:to="loc_us-gaap_ClassOfStockDomain_204410ba-923a-42c3-91d1-fb3eb043a29f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ClassOfStockDomain_c69fff80-8080-44f2-bc37-17e23d1b6bde" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_ClassOfStockDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementClassOfStockAxis_204410ba-923a-42c3-91d1-fb3eb043a29f" xlink:to="loc_us-gaap_ClassOfStockDomain_c69fff80-8080-44f2-bc37-17e23d1b6bde" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_ee51ec82-69af-4fc1-bb13-00f688a95542" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ClassOfStockDomain_c69fff80-8080-44f2-bc37-17e23d1b6bde" xlink:to="loc_us-gaap_CommonStockMember_ee51ec82-69af-4fc1-bb13-00f688a95542" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" xlink:type="simple" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-roles-2021-01-31.xsd#eedm"/>
  <link:definitionLink xlink:role="http://fasb.org/us-gaap/role/eedm/ExtensibleEnumerationLists" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsAbstract" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AssetsAbstract"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherAssetsNoncurrent" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_OtherAssetsNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract" xlink:to="loc_us-gaap_OtherAssetsNoncurrent" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_LiabilitiesCurrentAbstract"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccruedLiabilitiesCurrent" xlink:href="https://xbrl.fasb.org/us-gaap/2021/elts/us-gaap-2021-01-31.xsd#us-gaap_AccruedLiabilitiesCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract" xlink:to="loc_us-gaap_AccruedLiabilitiesCurrent" xlink:type="arc" order="0"/>
  </link:definitionLink>
</link:linkbase>
