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

<!--XBRL Document Created with the Workiva Platform-->
<!--Copyright 2026 Workiva-->
<!--r:019c9261-af25-7c7f-be64-6d6b12ae249c,g:22bfebc7-9232-4bef-aa5d-7f27abd07d41-->
<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/CondensedConsolidatedStatementsofIncome" xlink:type="simple" xlink:href="ftnt-20260331.xsd#CondensedConsolidatedStatementsofIncome"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/CondensedConsolidatedStatementsofIncome" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019c9261-b064-7fa5-bec3-114518cdc1c1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019c9261-b064-7637-b262-89a75bb0f1d7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019c9261-b064-7fa5-bec3-114518cdc1c1" xlink:to="loc_us-gaap_StatementTable_019c9261-b064-7637-b262-89a75bb0f1d7" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019c9261-b064-716c-83ee-e4e41ab58dfd" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019c9261-b064-7637-b262-89a75bb0f1d7" xlink:to="loc_srt_ProductOrServiceAxis_019c9261-b064-716c-83ee-e4e41ab58dfd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019c9261-b064-716c-83ee-e4e41ab58dfd_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019c9261-b064-716c-83ee-e4e41ab58dfd" xlink:to="loc_srt_ProductsAndServicesDomain_019c9261-b064-716c-83ee-e4e41ab58dfd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019c9261-b064-7ef5-9fa5-ba6ff5e1809f" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019c9261-b064-716c-83ee-e4e41ab58dfd" xlink:to="loc_srt_ProductsAndServicesDomain_019c9261-b064-7ef5-9fa5-ba6ff5e1809f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductMember_019c9261-b064-7229-aaa4-fee84707877a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProductMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019c9261-b064-7ef5-9fa5-ba6ff5e1809f" xlink:to="loc_us-gaap_ProductMember_019c9261-b064-7229-aaa4-fee84707877a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServiceMember_019c9261-b064-71d5-821d-dc04b39d5ba5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ServiceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019c9261-b064-7ef5-9fa5-ba6ff5e1809f" xlink:to="loc_us-gaap_ServiceMember_019c9261-b064-71d5-821d-dc04b39d5ba5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerAbstract_019c9261-b064-799f-bf5f-7369376448a7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c9261-b064-7fa5-bec3-114518cdc1c1" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerAbstract_019c9261-b064-799f-bf5f-7369376448a7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019c9261-b064-7ea2-821b-4ca2754c5052" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueFromContractWithCustomerAbstract_019c9261-b064-799f-bf5f-7369376448a7" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019c9261-b064-7ea2-821b-4ca2754c5052" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServicesSoldAbstract_019c9261-b064-7d6a-87e9-fc4fa245ded2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CostOfGoodsAndServicesSoldAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c9261-b064-7fa5-bec3-114518cdc1c1" xlink:to="loc_us-gaap_CostOfGoodsAndServicesSoldAbstract_019c9261-b064-7d6a-87e9-fc4fa245ded2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServicesSold_019c9261-b064-76c8-b3c2-74e3c02d1323" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CostOfGoodsAndServicesSold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostOfGoodsAndServicesSoldAbstract_019c9261-b064-7d6a-87e9-fc4fa245ded2" xlink:to="loc_us-gaap_CostOfGoodsAndServicesSold_019c9261-b064-76c8-b3c2-74e3c02d1323" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GrossProfitAbstract_019c9261-b064-7121-91b0-78c4869622a1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GrossProfitAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c9261-b064-7fa5-bec3-114518cdc1c1" xlink:to="loc_us-gaap_GrossProfitAbstract_019c9261-b064-7121-91b0-78c4869622a1" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GrossProfit_019c9261-b064-767c-a878-b6209e1d72e5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GrossProfit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GrossProfitAbstract_019c9261-b064-7121-91b0-78c4869622a1" xlink:to="loc_us-gaap_GrossProfit_019c9261-b064-767c-a878-b6209e1d72e5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpensesAbstract_019c9261-b064-7a9a-93ea-343d790c5777" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingExpensesAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c9261-b064-7fa5-bec3-114518cdc1c1" xlink:to="loc_us-gaap_OperatingExpensesAbstract_019c9261-b064-7a9a-93ea-343d790c5777" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_019c9261-b064-78cb-834d-aca09d33d45c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_019c9261-b064-7a9a-93ea-343d790c5777" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_019c9261-b064-78cb-834d-aca09d33d45c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingAndMarketingExpense_019c9261-b064-713a-a643-2c03032189b4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SellingAndMarketingExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_019c9261-b064-7a9a-93ea-343d790c5777" xlink:to="loc_us-gaap_SellingAndMarketingExpense_019c9261-b064-713a-a643-2c03032189b4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpense_019c9261-b064-792d-b7d7-0c992d037e49" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_019c9261-b064-7a9a-93ea-343d790c5777" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpense_019c9261-b064-792d-b7d7-0c992d037e49" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_GainLossFromMutualCovenantNotToSueAgreementRightToUseExistingIntellectualProperty_019c9261-b064-76de-9461-45a1844c068b" xlink:href="ftnt-20260331.xsd#ftnt_GainLossFromMutualCovenantNotToSueAgreementRightToUseExistingIntellectualProperty"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_019c9261-b064-7a9a-93ea-343d790c5777" xlink:to="loc_ftnt_GainLossFromMutualCovenantNotToSueAgreementRightToUseExistingIntellectualProperty_019c9261-b064-76de-9461-45a1844c068b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingExpenses_019c9261-b064-78f0-9456-e170b29354f5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingExpenses"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_019c9261-b064-7a9a-93ea-343d790c5777" xlink:to="loc_us-gaap_OperatingExpenses_019c9261-b064-78f0-9456-e170b29354f5" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingIncomeLoss_019c9261-b064-7945-bebb-00a11a123d66" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c9261-b064-7fa5-bec3-114518cdc1c1" xlink:to="loc_us-gaap_OperatingIncomeLoss_019c9261-b064-7945-bebb-00a11a123d66" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InvestmentIncomeNet_019c9261-b064-732e-a3fc-0bb79e29de7b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InvestmentIncomeNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c9261-b064-7fa5-bec3-114518cdc1c1" xlink:to="loc_us-gaap_InvestmentIncomeNet_019c9261-b064-732e-a3fc-0bb79e29de7b" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseNonoperating_019c9261-b064-75de-bfd6-561dbafd9928" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseNonoperating"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c9261-b064-7fa5-bec3-114518cdc1c1" xlink:to="loc_us-gaap_InterestExpenseNonoperating_019c9261-b064-75de-bfd6-561dbafd9928" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_019c9261-b064-7b83-accd-aed8af56b7c6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherNonoperatingIncomeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c9261-b064-7fa5-bec3-114518cdc1c1" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_019c9261-b064-7b83-accd-aed8af56b7c6" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments_019c9261-b064-7852-8e05-636ef2bd6b56" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c9261-b064-7fa5-bec3-114518cdc1c1" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments_019c9261-b064-7852-8e05-636ef2bd6b56" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_019c9261-b064-75ec-96ca-6a77ef74926d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c9261-b064-7fa5-bec3-114518cdc1c1" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_019c9261-b064-75ec-96ca-6a77ef74926d" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestments_019c9261-b064-7b23-a4bf-363255cc4cb4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c9261-b064-7fa5-bec3-114518cdc1c1" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestments_019c9261-b064-7b23-a4bf-363255cc4cb4" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019c9261-b064-78fc-a0dc-acbbea8aa542" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c9261-b064-7fa5-bec3-114518cdc1c1" xlink:to="loc_us-gaap_NetIncomeLoss_019c9261-b064-78fc-a0dc-acbbea8aa542" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareAbstract_019c9261-b064-7c36-bc3d-3387b4678bc8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c9261-b064-7fa5-bec3-114518cdc1c1" xlink:to="loc_us-gaap_EarningsPerShareAbstract_019c9261-b064-7c36-bc3d-3387b4678bc8" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_019c9261-b064-7b8d-8ccd-611ccb8b06d3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareAbstract_019c9261-b064-7c36-bc3d-3387b4678bc8" xlink:to="loc_us-gaap_EarningsPerShareBasic_019c9261-b064-7b8d-8ccd-611ccb8b06d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_019c9261-b064-7ad7-8acb-1bdff88b8b2b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareAbstract_019c9261-b064-7c36-bc3d-3387b4678bc8" xlink:to="loc_us-gaap_EarningsPerShareDiluted_019c9261-b064-7ad7-8acb-1bdff88b8b2b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_019c9261-b064-79d5-a724-ad4891b06253" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c9261-b064-7fa5-bec3-114518cdc1c1" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_019c9261-b064-79d5-a724-ad4891b06253" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_019c9261-b064-7ba7-ba24-cac7a6a612e9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_019c9261-b064-79d5-a724-ad4891b06253" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_019c9261-b064-7ba7-ba24-cac7a6a612e9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_019c9261-b064-7307-b394-8f7fd2310d48" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_019c9261-b064-79d5-a724-ad4891b06253" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_019c9261-b064-7307-b394-8f7fd2310d48" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/CondensedConsolidatedStatementsofStockholdersEquityDeficit" xlink:type="simple" xlink:href="ftnt-20260331.xsd#CondensedConsolidatedStatementsofStockholdersEquityDeficit"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/CondensedConsolidatedStatementsofStockholdersEquityDeficit" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementLineItems_019c9261-b064-7c27-b50e-36e0c59894ca" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementTable_019c9261-b064-7224-aef4-e1c7fb99cb96" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_019c9261-b064-7c27-b50e-36e0c59894ca" xlink:to="loc_us-gaap_StatementTable_019c9261-b064-7224-aef4-e1c7fb99cb96" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_019c9261-b064-744a-90fa-6f07114a12eb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementEquityComponentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_019c9261-b064-7224-aef4-e1c7fb99cb96" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_019c9261-b064-744a-90fa-6f07114a12eb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019c9261-b064-744a-90fa-6f07114a12eb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019c9261-b064-744a-90fa-6f07114a12eb" xlink:to="loc_us-gaap_EquityComponentDomain_019c9261-b064-744a-90fa-6f07114a12eb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityComponentDomain_019c9261-b064-7107-8b17-afc4b0a36dc2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityComponentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_019c9261-b064-744a-90fa-6f07114a12eb" xlink:to="loc_us-gaap_EquityComponentDomain_019c9261-b064-7107-8b17-afc4b0a36dc2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommonStockMember_019c9261-b064-75e3-beed-b487c94aaaa5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommonStockMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019c9261-b064-7107-8b17-afc4b0a36dc2" xlink:to="loc_us-gaap_CommonStockMember_019c9261-b064-75e3-beed-b487c94aaaa5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_019c9261-b064-7f57-9734-765e922c04a3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdditionalPaidInCapitalMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019c9261-b064-7107-8b17-afc4b0a36dc2" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_019c9261-b064-7f57-9734-765e922c04a3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019c9261-b064-7319-801d-e246c1a1d2c4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019c9261-b064-7107-8b17-afc4b0a36dc2" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_019c9261-b064-7319-801d-e246c1a1d2c4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RetainedEarningsMember_019c9261-b064-77e0-900b-e1b54d27184c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RetainedEarningsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_019c9261-b064-7107-8b17-afc4b0a36dc2" xlink:to="loc_us-gaap_RetainedEarningsMember_019c9261-b064-77e0-900b-e1b54d27184c" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c9261-b064-7efd-ac20-86c8c884340f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_019c9261-b064-7c27-b50e-36e0c59894ca" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c9261-b064-7efd-ac20-86c8c884340f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesIssued_019c9261-b064-7e63-b61c-3e64fac1779e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharesIssued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c9261-b064-7efd-ac20-86c8c884340f" xlink:to="loc_us-gaap_SharesIssued_019c9261-b064-7e63-b61c-3e64fac1779e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019c9261-b064-76a2-a3e5-caedf5f1a7aa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c9261-b064-7efd-ac20-86c8c884340f" xlink:to="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019c9261-b064-76a2-a3e5-caedf5f1a7aa" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_StockIssuedDuringPeriodSharesEquityIncentivePlans_019c9261-b064-7207-a216-a568983ca5bf" xlink:href="ftnt-20260331.xsd#ftnt_StockIssuedDuringPeriodSharesEquityIncentivePlans"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c9261-b064-7efd-ac20-86c8c884340f" xlink:to="loc_ftnt_StockIssuedDuringPeriodSharesEquityIncentivePlans_019c9261-b064-7207-a216-a568983ca5bf" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_StockIssuedDuringPeriodValueEquityIncentivePlansNetofTaxesWithheld_019c9261-b064-790e-b665-f19662b003d0" xlink:href="ftnt-20260331.xsd#ftnt_StockIssuedDuringPeriodValueEquityIncentivePlansNetofTaxesWithheld"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c9261-b064-7efd-ac20-86c8c884340f" xlink:to="loc_ftnt_StockIssuedDuringPeriodValueEquityIncentivePlansNetofTaxesWithheld_019c9261-b064-790e-b665-f19662b003d0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_019c9261-b064-7e87-a7cb-eab86e43d0f4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c9261-b064-7efd-ac20-86c8c884340f" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_019c9261-b064-7e87-a7cb-eab86e43d0f4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_019c9261-b064-7cfc-934e-bac6fc187d6f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c9261-b064-7efd-ac20-86c8c884340f" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_019c9261-b064-7cfc-934e-bac6fc187d6f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareRepurchaseProgramExciseTax_019c9261-b064-7e77-a3f2-29bf710f6904" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareRepurchaseProgramExciseTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c9261-b064-7efd-ac20-86c8c884340f" xlink:to="loc_us-gaap_ShareRepurchaseProgramExciseTax_019c9261-b064-7e77-a3f2-29bf710f6904" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019c9261-b064-7215-9354-5911fbf2b49b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c9261-b064-7efd-ac20-86c8c884340f" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_019c9261-b064-7215-9354-5911fbf2b49b" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_019c9261-b064-7444-abc5-96fc741b9443" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c9261-b064-7efd-ac20-86c8c884340f" xlink:to="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_019c9261-b064-7444-abc5-96fc741b9443" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_019c9261-b064-7236-bf72-06358a00b2ac" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c9261-b064-7efd-ac20-86c8c884340f" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossForeignCurrencyTransactionAndTranslationAdjustmentNetOfTax_019c9261-b064-7236-bf72-06358a00b2ac" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019c9261-b064-7a6e-8f65-b67160a7edab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_019c9261-b064-7efd-ac20-86c8c884340f" xlink:to="loc_us-gaap_NetIncomeLoss_019c9261-b064-7a6e-8f65-b67160a7edab" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesIssued_019c9261-b064-7830-8cf9-2ed3b0d1c84c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharesIssued"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest_019c9261-b064-7f30-aeb4-774e0a7ada81" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockholdersEquityIncludingPortionAttributableToNoncontrollingInterest"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/RevenueRecognitionDisaggregatedRevenuesDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#RevenueRecognitionDisaggregatedRevenuesDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/RevenueRecognitionDisaggregatedRevenuesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019c9261-b11f-743b-ae56-aca133b7e53c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019c9261-b11f-7ffd-8cdf-3462a1be7be0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019c9261-b11f-743b-ae56-aca133b7e53c" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019c9261-b11f-7ffd-8cdf-3462a1be7be0" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019c9261-b11f-71cd-b367-56929d1e36fd" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019c9261-b11f-7ffd-8cdf-3462a1be7be0" xlink:to="loc_srt_ProductOrServiceAxis_019c9261-b11f-71cd-b367-56929d1e36fd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019c9261-b11f-71cd-b367-56929d1e36fd_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019c9261-b11f-71cd-b367-56929d1e36fd" xlink:to="loc_srt_ProductsAndServicesDomain_019c9261-b11f-71cd-b367-56929d1e36fd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019c9261-b11f-7b12-a86c-861d2a10dbd7" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019c9261-b11f-71cd-b367-56929d1e36fd" xlink:to="loc_srt_ProductsAndServicesDomain_019c9261-b11f-7b12-a86c-861d2a10dbd7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductMember_019c9261-b11f-7857-9a96-221b73151297" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProductMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019c9261-b11f-7b12-a86c-861d2a10dbd7" xlink:to="loc_us-gaap_ProductMember_019c9261-b11f-7857-9a96-221b73151297" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServiceMember_019c9261-b11f-7f80-93ea-7b3d96968b90" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ServiceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019c9261-b11f-7b12-a86c-861d2a10dbd7" xlink:to="loc_us-gaap_ServiceMember_019c9261-b11f-7f80-93ea-7b3d96968b90" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_SecuritySubscriptionMember_019c9261-b11f-7f14-bb6a-59ffe6bb376b" xlink:href="ftnt-20260331.xsd#ftnt_SecuritySubscriptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ServiceMember_019c9261-b11f-7f80-93ea-7b3d96968b90" xlink:to="loc_ftnt_SecuritySubscriptionMember_019c9261-b11f-7f14-bb6a-59ffe6bb376b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_TechnicalSupportandOtherMember_019c9261-b11f-76dd-9970-4d8df802ea22" xlink:href="ftnt-20260331.xsd#ftnt_TechnicalSupportandOtherMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ServiceMember_019c9261-b11f-7f80-93ea-7b3d96968b90" xlink:to="loc_ftnt_TechnicalSupportandOtherMember_019c9261-b11f-76dd-9970-4d8df802ea22" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019c9261-b11f-7fe1-8955-13e7ac075d11" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019c9261-b11f-743b-ae56-aca133b7e53c" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019c9261-b11f-7fe1-8955-13e7ac075d11" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/RevenueRecognitionAdditionalInformationDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#RevenueRecognitionAdditionalInformationDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/RevenueRecognitionAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019c9261-b11f-7eac-afab-ff2bfd4bd1e4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019c9261-b11f-7a96-9eff-1b8b0ac9b9c1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019c9261-b11f-7eac-afab-ff2bfd4bd1e4" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019c9261-b11f-7a96-9eff-1b8b0ac9b9c1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019c9261-b11f-7fa3-a3fe-e1c2b2a159fc" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019c9261-b11f-7a96-9eff-1b8b0ac9b9c1" xlink:to="loc_srt_ProductOrServiceAxis_019c9261-b11f-7fa3-a3fe-e1c2b2a159fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019c9261-b11f-7fa3-a3fe-e1c2b2a159fc_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019c9261-b11f-7fa3-a3fe-e1c2b2a159fc" xlink:to="loc_srt_ProductsAndServicesDomain_019c9261-b11f-7fa3-a3fe-e1c2b2a159fc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019c9261-b11f-7992-9c62-98ac2caff9e5" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019c9261-b11f-7fa3-a3fe-e1c2b2a159fc" xlink:to="loc_srt_ProductsAndServicesDomain_019c9261-b11f-7992-9c62-98ac2caff9e5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServiceMember_019c9261-b11f-79d2-9976-3f496bd84a24" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ServiceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019c9261-b11f-7992-9c62-98ac2caff9e5" xlink:to="loc_us-gaap_ServiceMember_019c9261-b11f-79d2-9976-3f496bd84a24" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_019c9261-b11f-788c-85b6-95bb6f38f176" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019c9261-b11f-7a96-9eff-1b8b0ac9b9c1" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_019c9261-b11f-788c-85b6-95bb6f38f176" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_019c9261-b11f-758e-b3ce-a4a4ab852750" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerLiabilityRevenueRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019c9261-b11f-7eac-afab-ff2bfd4bd1e4" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_019c9261-b11f-758e-b3ce-a4a4ab852750" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostAmortization_019c9261-b11f-7e42-bceb-4988fecb0578" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalizedContractCostAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019c9261-b11f-7eac-afab-ff2bfd4bd1e4" xlink:to="loc_us-gaap_CapitalizedContractCostAmortization_019c9261-b11f-7e42-bceb-4988fecb0578" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/RevenueRecognitionAdditionalInformationDetails_1" xlink:type="simple" xlink:href="ftnt-20260331.xsd#RevenueRecognitionAdditionalInformationDetails_1"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/RevenueRecognitionAdditionalInformationDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_019c9261-b11f-7eac-afab-ff2bfd4bd1e4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_019c9261-b11f-7a96-9eff-1b8b0ac9b9c1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DisaggregationOfRevenueTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019c9261-b11f-7eac-afab-ff2bfd4bd1e4" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_019c9261-b11f-7a96-9eff-1b8b0ac9b9c1" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019c9261-b11f-7fa3-a3fe-e1c2b2a159fc" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_019c9261-b11f-7a96-9eff-1b8b0ac9b9c1" xlink:to="loc_srt_ProductOrServiceAxis_019c9261-b11f-7fa3-a3fe-e1c2b2a159fc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019c9261-b11f-7fa3-a3fe-e1c2b2a159fc_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019c9261-b11f-7fa3-a3fe-e1c2b2a159fc" xlink:to="loc_srt_ProductsAndServicesDomain_019c9261-b11f-7fa3-a3fe-e1c2b2a159fc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019c9261-b11f-7992-9c62-98ac2caff9e5" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019c9261-b11f-7fa3-a3fe-e1c2b2a159fc" xlink:to="loc_srt_ProductsAndServicesDomain_019c9261-b11f-7992-9c62-98ac2caff9e5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServiceMember_019c9261-b11f-79d2-9976-3f496bd84a24" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ServiceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019c9261-b11f-7992-9c62-98ac2caff9e5" xlink:to="loc_us-gaap_ServiceMember_019c9261-b11f-79d2-9976-3f496bd84a24" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_019c9261-b11f-758e-b3ce-a4a4ab852750" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ContractWithCustomerLiabilityRevenueRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019c9261-b11f-7eac-afab-ff2bfd4bd1e4" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityRevenueRecognized_019c9261-b11f-758e-b3ce-a4a4ab852750" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CapitalizedContractCostAmortization_019c9261-b11f-7e42-bceb-4988fecb0578" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CapitalizedContractCostAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_019c9261-b11f-7eac-afab-ff2bfd4bd1e4" xlink:to="loc_us-gaap_CapitalizedContractCostAmortization_019c9261-b11f-7e42-bceb-4988fecb0578" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/RevenueRecognitionPerformanceObligationSatisfactionPeriodDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#RevenueRecognitionPerformanceObligationSatisfactionPeriodDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/RevenueRecognitionPerformanceObligationSatisfactionPeriodDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019c9261-b11f-7831-a9c4-f70997779aaa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019c9261-b11f-7a86-ae2e-49bae0fc506f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019c9261-b11f-7831-a9c4-f70997779aaa" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019c9261-b11f-7a86-ae2e-49bae0fc506f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_019c9261-b11f-747b-987a-80efb66b4eb9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019c9261-b11f-7a86-ae2e-49bae0fc506f" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_019c9261-b11f-747b-987a-80efb66b4eb9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_019c9261-b11f-7e3e-90cf-0e1815c2b509" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019c9261-b11f-7831-a9c4-f70997779aaa" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_019c9261-b11f-7e3e-90cf-0e1815c2b509" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019c9261-b11f-719e-b1ee-50e22934847a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019c9261-b11f-7831-a9c4-f70997779aaa" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019c9261-b11f-719e-b1ee-50e22934847a" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/RevenueRecognitionPerformanceObligationSatisfactionPeriodDetails_1" xlink:type="simple" xlink:href="ftnt-20260331.xsd#RevenueRecognitionPerformanceObligationSatisfactionPeriodDetails_1"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/RevenueRecognitionPerformanceObligationSatisfactionPeriodDetails_1" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019c9261-b11f-7831-a9c4-f70997779aaa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019c9261-b11f-7a86-ae2e-49bae0fc506f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019c9261-b11f-7831-a9c4-f70997779aaa" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_019c9261-b11f-7a86-ae2e-49bae0fc506f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligation_019c9261-b11f-7e3e-90cf-0e1815c2b509" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019c9261-b11f-7831-a9c4-f70997779aaa" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligation_019c9261-b11f-7e3e-90cf-0e1815c2b509" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019c9261-b11f-719e-b1ee-50e22934847a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_019c9261-b11f-7831-a9c4-f70997779aaa" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_019c9261-b11f-719e-b1ee-50e22934847a" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/FinancialInstrumentsandFairValueInvestmentsDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#FinancialInstrumentsandFairValueInvestmentsDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/FinancialInstrumentsandFairValueInvestmentsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c9261-b11f-73c9-b43b-f97470e91254" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019c9261-b11f-7513-8e4d-507c026ded59" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c9261-b11f-73c9-b43b-f97470e91254" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019c9261-b11f-7513-8e4d-507c026ded59" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019c9261-b11f-759a-9262-078718371238" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtSecuritiesAvailableForSaleTable_019c9261-b11f-7513-8e4d-507c026ded59" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019c9261-b11f-759a-9262-078718371238" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c9261-b11f-759a-9262-078718371238_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019c9261-b11f-759a-9262-078718371238" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c9261-b11f-759a-9262-078718371238_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c9261-b11f-7bfa-8a30-1122efbf56e7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019c9261-b11f-759a-9262-078718371238" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c9261-b11f-7bfa-8a30-1122efbf56e7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_019c9261-b11f-70b5-8898-caf78b5832f7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USTreasuryAndGovernmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c9261-b11f-7bfa-8a30-1122efbf56e7" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_019c9261-b11f-70b5-8898-caf78b5832f7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_019c9261-b11f-74eb-9c3e-2fe6e8122321" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c9261-b11f-7bfa-8a30-1122efbf56e7" xlink:to="loc_us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_019c9261-b11f-74eb-9c3e-2fe6e8122321" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019c9261-b11f-73de-a819-5ed7eb3f2d29" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c9261-b11f-7bfa-8a30-1122efbf56e7" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019c9261-b11f-73de-a819-5ed7eb3f2d29" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_CertificatesofDepositandTermDepositsMember_019c9261-b11f-7677-91ad-bbcd1cde0b2d" xlink:href="ftnt-20260331.xsd#ftnt_CertificatesofDepositandTermDepositsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c9261-b11f-7bfa-8a30-1122efbf56e7" xlink:to="loc_ftnt_CertificatesofDepositandTermDepositsMember_019c9261-b11f-7677-91ad-bbcd1cde0b2d" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_019c9261-b11f-768f-8598-5e5020354bed" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c9261-b11f-73c9-b43b-f97470e91254" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_019c9261-b11f-768f-8598-5e5020354bed" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019c9261-b11f-798f-a229-c078f0f22863" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c9261-b11f-73c9-b43b-f97470e91254" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_019c9261-b11f-798f-a229-c078f0f22863" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019c9261-b11f-7373-94c6-4605880b6210" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c9261-b11f-73c9-b43b-f97470e91254" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_019c9261-b11f-7373-94c6-4605880b6210" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019c9261-b11f-7133-8027-aef647ceb944" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c9261-b11f-73c9-b43b-f97470e91254" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019c9261-b11f-7133-8027-aef647ceb944" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent_019c9261-b11f-75a0-ba29-62e4a8155b2a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c9261-b11f-73c9-b43b-f97470e91254" xlink:to="loc_us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent_019c9261-b11f-75a0-ba29-62e4a8155b2a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesFvNi_019dcff3-27a5-7259-ba7e-7e9fc3a43700" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesFvNi"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c9261-b11f-73c9-b43b-f97470e91254" xlink:to="loc_us-gaap_EquitySecuritiesFvNi_019dcff3-27a5-7259-ba7e-7e9fc3a43700" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_DebtSecuritiesAvailableForSaleAndEquitySecurities_019c9261-b11f-7794-af7a-0c1a3b44305f" xlink:href="ftnt-20260331.xsd#ftnt_DebtSecuritiesAvailableForSaleAndEquitySecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c9261-b11f-73c9-b43b-f97470e91254" xlink:to="loc_ftnt_DebtSecuritiesAvailableForSaleAndEquitySecurities_019c9261-b11f-7794-af7a-0c1a3b44305f" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_019c9261-b11f-70ad-8223-4bbd4ecaa483" xlink:href="ftnt-20260331.xsd#ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c9261-b11f-73c9-b43b-f97470e91254" xlink:to="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_019c9261-b11f-70ad-8223-4bbd4ecaa483" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months_019c9261-b11f-78c3-9790-26fceb85f2aa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_019c9261-b11f-70ad-8223-4bbd4ecaa483" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12Months_019c9261-b11f-78c3-9790-26fceb85f2aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_019c9261-b11f-7f5b-9a61-09b94265f57e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_019c9261-b11f-70ad-8223-4bbd4ecaa483" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_019c9261-b11f-7f5b-9a61-09b94265f57e" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger_019c9261-b11f-704e-b95a-3b48b8f8ded3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_019c9261-b11f-70ad-8223-4bbd4ecaa483" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLonger_019c9261-b11f-704e-b95a-3b48b8f8ded3" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_019c9261-b11f-742e-acaf-01f11fee0c6a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_019c9261-b11f-70ad-8223-4bbd4ecaa483" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_019c9261-b11f-742e-acaf-01f11fee0c6a" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition_019c9261-b11f-7fe6-a846-c69e07390379" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_019c9261-b11f-70ad-8223-4bbd4ecaa483" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPosition_019c9261-b11f-7fe6-a846-c69e07390379" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_019c9261-b11f-75cd-8443-02a7b6b9b647" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_019c9261-b11f-70ad-8223-4bbd4ecaa483" xlink:to="loc_us-gaap_DebtSecuritiesAvailableForSaleUnrealizedLossPositionAccumulatedLoss_019c9261-b11f-75cd-8443-02a7b6b9b647" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_019c9261-b11f-78d4-9b4e-de8fb19bf572" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_019c9261-b11f-73c9-b43b-f97470e91254" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_019c9261-b11f-78d4-9b4e-de8fb19bf572" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue_019c9261-b11f-7806-8feb-a84193355e71" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_019c9261-b11f-78d4-9b4e-de8fb19bf572" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue_019c9261-b11f-7806-8feb-a84193355e71" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_AvailableForSaleSecuritiesDebtMaturitiesAfterOneYearFairValue_019c9261-b11f-7fe0-a14a-b414788f37fe" xlink:href="ftnt-20260331.xsd#ftnt_AvailableForSaleSecuritiesDebtMaturitiesAfterOneYearFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_019c9261-b11f-78d4-9b4e-de8fb19bf572" xlink:to="loc_ftnt_AvailableForSaleSecuritiesDebtMaturitiesAfterOneYearFairValue_019c9261-b11f-7fe0-a14a-b414788f37fe" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019c9261-b11f-7a50-99d4-d8ab2db745bf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/FinancialInstrumentsandFairValueFairValueMeasurementsDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#FinancialInstrumentsandFairValueFairValueMeasurementsDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/FinancialInstrumentsandFairValueFairValueMeasurementsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019c9261-b11f-778e-bcdf-dea230b0ead5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019c9261-b11f-7782-9a96-c3be42ed3557" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019c9261-b11f-778e-bcdf-dea230b0ead5" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019c9261-b11f-7782-9a96-c3be42ed3557" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019c9261-b11f-7e18-adff-10711e81314f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByMeasurementFrequencyAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019c9261-b11f-7782-9a96-c3be42ed3557" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019c9261-b11f-7e18-adff-10711e81314f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_019c9261-b11f-7e18-adff-10711e81314f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019c9261-b11f-7e18-adff-10711e81314f" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_019c9261-b11f-7e18-adff-10711e81314f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_019c9261-b11f-7b29-a488-a551719c5149" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementFrequencyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_019c9261-b11f-7e18-adff-10711e81314f" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_019c9261-b11f-7b29-a488-a551719c5149" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_019c9261-b11f-7719-bd4e-1d7dc257d8d7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsRecurringMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_019c9261-b11f-7b29-a488-a551719c5149" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_019c9261-b11f-7719-bd4e-1d7dc257d8d7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByMeasurementBasisAxis_019c9261-b11f-7bf0-89ae-e62d05f8c83f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByMeasurementBasisAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019c9261-b11f-7782-9a96-c3be42ed3557" xlink:to="loc_us-gaap_FairValueByMeasurementBasisAxis_019c9261-b11f-7bf0-89ae-e62d05f8c83f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019c9261-b11f-7bf0-89ae-e62d05f8c83f_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_019c9261-b11f-7bf0-89ae-e62d05f8c83f" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019c9261-b11f-7bf0-89ae-e62d05f8c83f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019c9261-b11f-705a-93d9-125b94066ba7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueDisclosureItemAmountsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_019c9261-b11f-7bf0-89ae-e62d05f8c83f" xlink:to="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019c9261-b11f-705a-93d9-125b94066ba7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019c9261-b11f-7067-bf66-039de7ce209a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_019c9261-b11f-705a-93d9-125b94066ba7" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019c9261-b11f-7067-bf66-039de7ce209a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_019c9261-b11f-723f-90c7-c8dcd80411ff" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_019c9261-b11f-7067-bf66-039de7ce209a" xlink:to="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_019c9261-b11f-723f-90c7-c8dcd80411ff" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FinancialInstrumentAxis_019c9261-b11f-79b6-bdf5-541a9d184d53" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FinancialInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019c9261-b11f-7782-9a96-c3be42ed3557" xlink:to="loc_us-gaap_FinancialInstrumentAxis_019c9261-b11f-79b6-bdf5-541a9d184d53" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c9261-b11f-79b6-bdf5-541a9d184d53_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019c9261-b11f-79b6-bdf5-541a9d184d53" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c9261-b11f-79b6-bdf5-541a9d184d53_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c9261-b11f-79ea-bde4-6db49ad98159" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_019c9261-b11f-79b6-bdf5-541a9d184d53" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c9261-b11f-79ea-bde4-6db49ad98159" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_MoneyMarketFundsExcludingCommercialPaperMember_019c9261-b11f-7e83-b4ca-bb26d2ef5133" xlink:href="ftnt-20260331.xsd#ftnt_MoneyMarketFundsExcludingCommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c9261-b11f-79ea-bde4-6db49ad98159" xlink:to="loc_ftnt_MoneyMarketFundsExcludingCommercialPaperMember_019c9261-b11f-7e83-b4ca-bb26d2ef5133" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_019c9261-b11f-7941-902e-b416178de9cd" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_USTreasuryAndGovernmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c9261-b11f-79ea-bde4-6db49ad98159" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_019c9261-b11f-7941-902e-b416178de9cd" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperMember_019c9261-b11f-77c4-92ef-6c75c2a4fe05" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialPaperMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c9261-b11f-79ea-bde4-6db49ad98159" xlink:to="loc_us-gaap_CommercialPaperMember_019c9261-b11f-77c4-92ef-6c75c2a4fe05" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_019c9261-b11f-73f0-944d-d823d3e5f1d9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CorporateDebtSecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c9261-b11f-79ea-bde4-6db49ad98159" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_019c9261-b11f-73f0-944d-d823d3e5f1d9" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_019c9261-b11f-74b3-bf4e-af284a4da56c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c9261-b11f-79ea-bde4-6db49ad98159" xlink:to="loc_us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_019c9261-b11f-74b3-bf4e-af284a4da56c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_CertificatesofDepositandTermDepositsMember_019c9261-b11f-7642-8e31-fc7654e551cf" xlink:href="ftnt-20260331.xsd#ftnt_CertificatesofDepositandTermDepositsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c9261-b11f-79ea-bde4-6db49ad98159" xlink:to="loc_ftnt_CertificatesofDepositandTermDepositsMember_019c9261-b11f-7642-8e31-fc7654e551cf" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesMember_019c9261-b11f-7d70-a9e8-201cab774e47" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_019c9261-b11f-79ea-bde4-6db49ad98159" xlink:to="loc_us-gaap_EquitySecuritiesMember_019c9261-b11f-7d70-a9e8-201cab774e47" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019c9261-b11f-74ec-bb28-4c6591dcafab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019c9261-b11f-7782-9a96-c3be42ed3557" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019c9261-b11f-74ec-bb28-4c6591dcafab" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c9261-b11f-74ec-bb28-4c6591dcafab_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019c9261-b11f-74ec-bb28-4c6591dcafab" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c9261-b11f-74ec-bb28-4c6591dcafab_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c9261-b11f-7ec0-8d5b-7a2f0bcb035e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019c9261-b11f-74ec-bb28-4c6591dcafab" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c9261-b11f-7ec0-8d5b-7a2f0bcb035e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_019c9261-b11f-780b-9b87-9c1d84a3ccbe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel1Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c9261-b11f-7ec0-8d5b-7a2f0bcb035e" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_019c9261-b11f-780b-9b87-9c1d84a3ccbe" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019c9261-b11f-76ee-b152-efbf71c9b11f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c9261-b11f-7ec0-8d5b-7a2f0bcb035e" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019c9261-b11f-76ee-b152-efbf71c9b11f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_019c9261-b11f-7e8d-8ed7-47d4eb77cc97" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel3Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c9261-b11f-7ec0-8d5b-7a2f0bcb035e" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_019c9261-b11f-7e8d-8ed7-47d4eb77cc97" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_019c9261-b11f-7429-8d9e-ce90b17bba17" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_019c9261-b11f-7782-9a96-c3be42ed3557" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_019c9261-b11f-7429-8d9e-ce90b17bba17" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_019c9261-b11f-7429-8d9e-ce90b17bba17_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_019c9261-b11f-7429-8d9e-ce90b17bba17" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_019c9261-b11f-7429-8d9e-ce90b17bba17_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_019c9261-b11f-7bc0-b44c-84e924f58afb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BalanceSheetLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_019c9261-b11f-7429-8d9e-ce90b17bba17" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_019c9261-b11f-7bc0-b44c-84e924f58afb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_CashEquivalentsLineItemMember_019c9261-b11f-71a5-a31a-f23ca0b211d7" xlink:href="ftnt-20260331.xsd#ftnt_CashEquivalentsLineItemMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_019c9261-b11f-7bc0-b44c-84e924f58afb" xlink:to="loc_ftnt_CashEquivalentsLineItemMember_019c9261-b11f-71a5-a31a-f23ca0b211d7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ShortTermInvestmentsLineItemMember_019c9261-b11f-7fa2-a83a-e5bced50f1ef" xlink:href="ftnt-20260331.xsd#ftnt_ShortTermInvestmentsLineItemMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_019c9261-b11f-7bc0-b44c-84e924f58afb" xlink:to="loc_ftnt_ShortTermInvestmentsLineItemMember_019c9261-b11f-7fa2-a83a-e5bced50f1ef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_LongTermInvestmentsLineItemMember_019c9261-b11f-7e8b-9cbf-e887a3fee472" xlink:href="ftnt-20260331.xsd#ftnt_LongTermInvestmentsLineItemMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_019c9261-b11f-7bc0-b44c-84e924f58afb" xlink:to="loc_ftnt_LongTermInvestmentsLineItemMember_019c9261-b11f-7e8b-9cbf-e887a3fee472" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019c9261-b11f-7ea3-8503-8eda19e48b3f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019c9261-b11f-778e-bcdf-dea230b0ead5" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_019c9261-b11f-7ea3-8503-8eda19e48b3f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent_019c9261-b11f-7a75-92a8-58ef4f3e8bba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019c9261-b11f-778e-bcdf-dea230b0ead5" xlink:to="loc_us-gaap_EquitySecuritiesFvNiCurrentAndNoncurrent_019c9261-b11f-7a75-92a8-58ef4f3e8bba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_DebtSecuritiesAvailableForSaleAndEquitySecurities_019c9261-b11f-778b-9c54-c63944afbdbe" xlink:href="ftnt-20260331.xsd#ftnt_DebtSecuritiesAvailableForSaleAndEquitySecurities"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019c9261-b11f-778e-bcdf-dea230b0ead5" xlink:to="loc_ftnt_DebtSecuritiesAvailableForSaleAndEquitySecurities_019c9261-b11f-778b-9c54-c63944afbdbe" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_019c9261-b11f-78ae-97f4-f7d4296b76d5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_019c9261-b11f-778e-bcdf-dea230b0ead5" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_019c9261-b11f-78ae-97f4-f7d4296b76d5" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/PropertyandEquipmentNetDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#PropertyandEquipmentNetDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/PropertyandEquipmentNetDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019c9261-b11f-7583-9975-763a6bb74f21" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019c9261-b11f-77ad-aa09-69105f440191" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019c9261-b11f-7583-9975-763a6bb74f21" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019c9261-b11f-77ad-aa09-69105f440191" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019c9261-b11f-7f65-b3fa-6d27db3473d7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019c9261-b11f-77ad-aa09-69105f440191" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019c9261-b11f-7f65-b3fa-6d27db3473d7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019c9261-b11f-7f65-b3fa-6d27db3473d7_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019c9261-b11f-7f65-b3fa-6d27db3473d7" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019c9261-b11f-7f65-b3fa-6d27db3473d7_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019c9261-b11f-70c9-b4fe-6b419eff3858" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_019c9261-b11f-7f65-b3fa-6d27db3473d7" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019c9261-b11f-70c9-b4fe-6b419eff3858" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LandMember_019c9261-b11f-7aa5-a6c1-e9fd623d631a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LandMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019c9261-b11f-70c9-b4fe-6b419eff3858" xlink:to="loc_us-gaap_LandMember_019c9261-b11f-7aa5-a6c1-e9fd623d631a" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BuildingAndBuildingImprovementsMember_019c9261-b11f-7aea-8892-94e17b7a725e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BuildingAndBuildingImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019c9261-b11f-70c9-b4fe-6b419eff3858" xlink:to="loc_us-gaap_BuildingAndBuildingImprovementsMember_019c9261-b11f-7aea-8892-94e17b7a725e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ComputerEquipmentMember_019c9261-b11f-7780-b8dd-cd18c62b1078" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ComputerEquipmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019c9261-b11f-70c9-b4fe-6b419eff3858" xlink:to="loc_us-gaap_ComputerEquipmentMember_019c9261-b11f-7780-b8dd-cd18c62b1078" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LeaseholdsAndLeaseholdImprovementsMember_019c9261-b11f-790c-a296-07f1452647a6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LeaseholdsAndLeaseholdImprovementsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019c9261-b11f-70c9-b4fe-6b419eff3858" xlink:to="loc_us-gaap_LeaseholdsAndLeaseholdImprovementsMember_019c9261-b11f-790c-a296-07f1452647a6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_EvaluationUnitsMember_019c9261-b11f-76e1-a4d8-4805751d50d6" xlink:href="ftnt-20260331.xsd#ftnt_EvaluationUnitsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019c9261-b11f-70c9-b4fe-6b419eff3858" xlink:to="loc_ftnt_EvaluationUnitsMember_019c9261-b11f-76e1-a4d8-4805751d50d6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_019c9261-b11f-7e08-a265-fedd24fe2421" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FurnitureAndFixturesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019c9261-b11f-70c9-b4fe-6b419eff3858" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_019c9261-b11f-7e08-a265-fedd24fe2421" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConstructionInProgressMember_019c9261-b11f-7dea-8bc5-da5c940aea89" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConstructionInProgressMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_019c9261-b11f-70c9-b4fe-6b419eff3858" xlink:to="loc_us-gaap_ConstructionInProgressMember_019c9261-b11f-7dea-8bc5-da5c940aea89" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetAcquisitionAxis_019dd02d-0633-7fc6-9058-846d09b525ca" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_019c9261-b11f-77ad-aa09-69105f440191" xlink:to="loc_us-gaap_AssetAcquisitionAxis_019dd02d-0633-7fc6-9058-846d09b525ca" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetAcquisitionDomain_019dd02d-0633-7fc6-9058-846d09b525ca_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetAcquisitionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AssetAcquisitionAxis_019dd02d-0633-7fc6-9058-846d09b525ca" xlink:to="loc_us-gaap_AssetAcquisitionDomain_019dd02d-0633-7fc6-9058-846d09b525ca_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetAcquisitionDomain_019dd02d-0633-709a-b648-dd6bf4eb6a21" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetAcquisitionDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AssetAcquisitionAxis_019dd02d-0633-7fc6-9058-846d09b525ca" xlink:to="loc_us-gaap_AssetAcquisitionDomain_019dd02d-0633-709a-b648-dd6bf4eb6a21" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_RealEstatePropertyMember_019dd02e-29b5-74ad-88a9-528c02d7e99f" xlink:href="ftnt-20260331.xsd#ftnt_RealEstatePropertyMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetAcquisitionDomain_019dd02d-0633-709a-b648-dd6bf4eb6a21" xlink:to="loc_ftnt_RealEstatePropertyMember_019dd02e-29b5-74ad-88a9-528c02d7e99f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract_019c9261-b11f-720e-b91d-5c79507e132b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019c9261-b11f-7583-9975-763a6bb74f21" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract_019c9261-b11f-720e-b91d-5c79507e132b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_019c9261-b11f-74a2-95e2-c1822dd3ab64" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract_019c9261-b11f-720e-b91d-5c79507e132b" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_019c9261-b11f-74a2-95e2-c1822dd3ab64" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_019c9261-b11f-7f5b-b2fa-c737606f8831" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract_019c9261-b11f-720e-b91d-5c79507e132b" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_019c9261-b11f-7f5b-b2fa-c737606f8831" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_019c9261-b11f-70ba-b1a6-de034318bc37" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract_019c9261-b11f-720e-b91d-5c79507e132b" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_019c9261-b11f-70ba-b1a6-de034318bc37" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireProductiveAssets_019dd02c-1835-77fa-ac22-ff6d235be16f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireProductiveAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019c9261-b11f-7583-9975-763a6bb74f21" xlink:to="loc_us-gaap_PaymentsToAcquireProductiveAssets_019dd02c-1835-77fa-ac22-ff6d235be16f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AssetAcquisitionConsiderationTransferred_019dd02d-7f24-7e2b-ba73-ec995499ed9a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AssetAcquisitionConsiderationTransferred"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019c9261-b11f-7583-9975-763a6bb74f21" xlink:to="loc_us-gaap_AssetAcquisitionConsiderationTransferred_019dd02d-7f24-7e2b-ba73-ec995499ed9a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Depreciation_019c9261-b11f-7d8f-bec1-c3ef7bc2f5eb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Depreciation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_019c9261-b11f-7583-9975-763a6bb74f21" xlink:to="loc_us-gaap_Depreciation_019c9261-b11f-7d8f-bec1-c3ef7bc2f5eb" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/BusinessCombinationsDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#BusinessCombinationsDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/BusinessCombinationsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_019c9261-b11f-7630-b30a-4a718ed81a72" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019c9261-b11f-7f9c-b264-4f6a27949f85" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019c9261-b11f-7630-b30a-4a718ed81a72" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019c9261-b11f-7f9c-b264-4f6a27949f85" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_019c9261-b11f-77cb-974a-c1eb96bf7299" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019c9261-b11f-7f9c-b264-4f6a27949f85" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_019c9261-b11f-77cb-974a-c1eb96bf7299" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019c9261-b11f-77cb-974a-c1eb96bf7299_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019c9261-b11f-77cb-974a-c1eb96bf7299" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019c9261-b11f-77cb-974a-c1eb96bf7299_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019c9261-b11f-7222-96c2-d36f757922c8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionAcquireeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_019c9261-b11f-77cb-974a-c1eb96bf7299" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019c9261-b11f-7222-96c2-d36f757922c8" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_LinksysMember_019c9261-b11f-7946-a309-9fcb00592484" xlink:href="ftnt-20260331.xsd#ftnt_LinksysMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019c9261-b11f-7222-96c2-d36f757922c8" xlink:to="loc_ftnt_LinksysMember_019c9261-b11f-7946-a309-9fcb00592484" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeriesOfIndividuallyImmaterialBusinessAcquisitionsMember_019c9261-b11f-7063-9b9d-0e282d9b8c87" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeriesOfIndividuallyImmaterialBusinessAcquisitionsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_019c9261-b11f-7222-96c2-d36f757922c8" xlink:to="loc_us-gaap_SeriesOfIndividuallyImmaterialBusinessAcquisitionsMember_019c9261-b11f-7063-9b9d-0e282d9b8c87" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019c9261-b11f-7edb-90c5-d25ed63a6c6f" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_019c9261-b11f-7f9c-b264-4f6a27949f85" xlink:to="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019c9261-b11f-7edb-90c5-d25ed63a6c6f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_019c9261-b11f-7edb-90c5-d25ed63a6c6f_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019c9261-b11f-7edb-90c5-d25ed63a6c6f" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_019c9261-b11f-7edb-90c5-d25ed63a6c6f_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_EquityMethodInvesteeNameDomain_019c9261-b11f-7982-b55b-dac71b1fb90d" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_EquityMethodInvesteeNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ScheduleOfEquityMethodInvestmentEquityMethodInvesteeNameAxis_019c9261-b11f-7edb-90c5-d25ed63a6c6f" xlink:to="loc_srt_EquityMethodInvesteeNameDomain_019c9261-b11f-7982-b55b-dac71b1fb90d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_LinksysMember_019c9261-b11f-7fe3-880b-d4f5468114ff" xlink:href="ftnt-20260331.xsd#ftnt_LinksysMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_EquityMethodInvesteeNameDomain_019c9261-b11f-7982-b55b-dac71b1fb90d" xlink:to="loc_ftnt_LinksysMember_019c9261-b11f-7fe3-880b-d4f5468114ff" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireEquityMethodInvestments_019c9261-b11f-73df-a8f5-bd64b79db52b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019c9261-b11f-7630-b30a-4a718ed81a72" xlink:to="loc_us-gaap_PaymentsToAcquireEquityMethodInvestments_019c9261-b11f-73df-a8f5-bd64b79db52b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_019c9261-b11f-78bc-ae48-654516cb86e5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestmentOwnershipPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019c9261-b11f-7630-b30a-4a718ed81a72" xlink:to="loc_us-gaap_EquityMethodInvestmentOwnershipPercentage_019c9261-b11f-78bc-ae48-654516cb86e5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesGross_019c9261-b11f-7cfe-9f24-9c9683f6c708" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PaymentsToAcquireBusinessesGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019c9261-b11f-7630-b30a-4a718ed81a72" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesGross_019c9261-b11f-7cfe-9f24-9c9683f6c708" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired_019c9261-b11f-71fb-8633-12876f69b168" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019c9261-b11f-7630-b30a-4a718ed81a72" xlink:to="loc_us-gaap_BusinessAcquisitionPercentageOfVotingInterestsAcquired_019c9261-b11f-71fb-8633-12876f69b168" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EquityMethodInvestmentsFairValueDisclosure_019c9261-b11f-795a-a2b0-7e55a457c5d4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EquityMethodInvestmentsFairValueDisclosure"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019c9261-b11f-7630-b30a-4a718ed81a72" xlink:to="loc_us-gaap_EquityMethodInvestmentsFairValueDisclosure_019c9261-b11f-795a-a2b0-7e55a457c5d4" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeLossFromEquityMethodInvestments_019c9261-b11f-7905-ac87-5bf0b4ff0705" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeLossFromEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019c9261-b11f-7630-b30a-4a718ed81a72" xlink:to="loc_us-gaap_IncomeLossFromEquityMethodInvestments_019c9261-b11f-7905-ac87-5bf0b4ff0705" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_019c9261-b11f-75d9-b268-57f9ab11ce42" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationConsiderationTransferred1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019c9261-b11f-7630-b30a-4a718ed81a72" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_019c9261-b11f-75d9-b268-57f9ab11ce42" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_019c9261-b11f-73ca-a7d4-d4ef099d1d80" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019c9261-b11f-7630-b30a-4a718ed81a72" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_019c9261-b11f-73ca-a7d4-d4ef099d1d80" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets_019c9261-b11f-7fe9-bdd7-f9742c413d2c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019c9261-b11f-7630-b30a-4a718ed81a72" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets_019c9261-b11f-7fe9-bdd7-f9742c413d2c" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxAssets_019c9261-b11f-71e1-94b8-e6b12e9b7e48" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxAssets"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019c9261-b11f-7630-b30a-4a718ed81a72" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxAssets_019c9261-b11f-71e1-94b8-e6b12e9b7e48" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInventory_019c9261-b11f-7ba1-9162-b16da0b5149f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInventory"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019c9261-b11f-7630-b30a-4a718ed81a72" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInventory_019c9261-b11f-7ba1-9162-b16da0b5149f" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_019c9261-b11f-78a8-b5f3-109d1d657f0c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019c9261-b11f-7630-b30a-4a718ed81a72" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_019c9261-b11f-78a8-b5f3-109d1d657f0c" xlink:type="arc" order="11"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherAssetsAndLiabilitiesNet_019c9261-b11f-7e1c-8d52-d2d540998b92" xlink:href="ftnt-20260331.xsd#ftnt_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherAssetsAndLiabilitiesNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019c9261-b11f-7630-b30a-4a718ed81a72" xlink:to="loc_ftnt_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedOtherAssetsAndLiabilitiesNet_019c9261-b11f-7e1c-8d52-d2d540998b92" xlink:type="arc" order="12"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount_019c9261-b11f-7109-b7e1-6723c6d67cd5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019c9261-b11f-7630-b30a-4a718ed81a72" xlink:to="loc_us-gaap_BusinessCombinationBargainPurchaseGainRecognizedAmount_019c9261-b11f-7109-b7e1-6723c6d67cd5" xlink:type="arc" order="13"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DeferredTaxAssetsEquityMethodInvestments_019c9261-b11f-7ac5-8e8b-39998e0f07d4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DeferredTaxAssetsEquityMethodInvestments"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019c9261-b11f-7630-b30a-4a718ed81a72" xlink:to="loc_us-gaap_DeferredTaxAssetsEquityMethodInvestments_019c9261-b11f-7ac5-8e8b-39998e0f07d4" xlink:type="arc" order="14"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_Goodwill_019c9261-b11f-79ef-b990-5968d3acd58c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_Goodwill"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019c9261-b11f-7630-b30a-4a718ed81a72" xlink:to="loc_us-gaap_Goodwill_019c9261-b11f-79ef-b990-5968d3acd58c" xlink:type="arc" order="15"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_019c9261-b11f-752c-b258-461bb4093588" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019c9261-b11f-7630-b30a-4a718ed81a72" xlink:to="loc_us-gaap_BusinessAcquisitionPurchasePriceAllocationGoodwillExpectedTaxDeductibleAmount_019c9261-b11f-752c-b258-461bb4093588" xlink:type="arc" order="16"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_BusinessAcquisitionGoodwillNotExpectedTaxDeductibleAmount_019c9261-b11f-7005-91f1-6c7cc5fd4469" xlink:href="ftnt-20260331.xsd#ftnt_BusinessAcquisitionGoodwillNotExpectedTaxDeductibleAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_019c9261-b11f-7630-b30a-4a718ed81a72" xlink:to="loc_ftnt_BusinessAcquisitionGoodwillNotExpectedTaxDeductibleAmount_019c9261-b11f-7005-91f1-6c7cc5fd4469" xlink:type="arc" order="17"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/GoodwillandOtherIntangibleAssetsNetOtherIntangibleAssetsNetDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#GoodwillandOtherIntangibleAssetsNetOtherIntangibleAssetsNetDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/GoodwillandOtherIntangibleAssetsNetOtherIntangibleAssetsNetDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019c9261-b11f-7253-9ffe-0115439c776a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019c9261-b11f-75bb-b12c-dc7becfd5969" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019c9261-b11f-7253-9ffe-0115439c776a" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019c9261-b11f-75bb-b12c-dc7becfd5969" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019c9261-b11f-756d-86d1-36670683cdcb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_019c9261-b11f-75bb-b12c-dc7becfd5969" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019c9261-b11f-756d-86d1-36670683cdcb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019c9261-b11f-756d-86d1-36670683cdcb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019c9261-b11f-756d-86d1-36670683cdcb" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019c9261-b11f-756d-86d1-36670683cdcb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019c9261-b11f-7635-b864-ab7a3fc1ae74" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_019c9261-b11f-756d-86d1-36670683cdcb" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019c9261-b11f-7635-b864-ab7a3fc1ae74" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_019c9261-b11f-7487-bf07-1a780e06f572" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DevelopedTechnologyRightsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019c9261-b11f-7635-b864-ab7a3fc1ae74" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_019c9261-b11f-7487-bf07-1a780e06f572" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerRelationshipsMember_019c9261-b11f-78cf-b1ed-74c034f4f7da" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CustomerRelationshipsMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019c9261-b11f-7635-b864-ab7a3fc1ae74" xlink:to="loc_us-gaap_CustomerRelationshipsMember_019c9261-b11f-78cf-b1ed-74c034f4f7da" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TradeNamesMember_019c9261-b11f-708a-a98f-4e2d38f81c71" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TradeNamesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019c9261-b11f-7635-b864-ab7a3fc1ae74" xlink:to="loc_us-gaap_TradeNamesMember_019c9261-b11f-708a-a98f-4e2d38f81c71" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_BacklogMember_019c9261-b11f-784c-a859-fdd6a498fd22" xlink:href="ftnt-20260331.xsd#ftnt_BacklogMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_019c9261-b11f-7635-b864-ab7a3fc1ae74" xlink:to="loc_ftnt_BacklogMember_019c9261-b11f-784c-a859-fdd6a498fd22" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_019c9261-b11f-7614-aee0-f9079dd2ba0f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019c9261-b11f-7253-9ffe-0115439c776a" xlink:to="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_019c9261-b11f-7614-aee0-f9079dd2ba0f" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_019c9261-b11f-72f8-9ee1-dc174ca6d81b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsNetAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_019c9261-b11f-7253-9ffe-0115439c776a" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_019c9261-b11f-72f8-9ee1-dc174ca6d81b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019c9261-b11f-784a-a3d8-cac6f9ad504e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsGross"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_019c9261-b11f-72f8-9ee1-dc174ca6d81b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_019c9261-b11f-784a-a3d8-cac6f9ad504e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019c9261-b11f-76bf-9980-2b5dae66286c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_019c9261-b11f-72f8-9ee1-dc174ca6d81b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_019c9261-b11f-76bf-9980-2b5dae66286c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019c9261-b11f-7cd1-9d69-de26b248d873" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FiniteLivedIntangibleAssetsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_019c9261-b11f-72f8-9ee1-dc174ca6d81b" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_019c9261-b11f-7cd1-9d69-de26b248d873" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/NetIncomePerShareCalculationofBasicandDilutedDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#NetIncomePerShareCalculationofBasicandDilutedDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/NetIncomePerShareCalculationofBasicandDilutedDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_EarningsPerShareLineItems_019c9261-b11f-7a65-8870-1adc0c580ba5" xlink:href="ftnt-20260331.xsd#ftnt_EarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ScheduleOfEarningsPerShareTable_019c9261-b11f-7b82-897a-61f6f438a8ba" xlink:href="ftnt-20260331.xsd#ftnt_ScheduleOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_ftnt_EarningsPerShareLineItems_019c9261-b11f-7a65-8870-1adc0c580ba5" xlink:to="loc_ftnt_ScheduleOfEarningsPerShareTable_019c9261-b11f-7b82-897a-61f6f438a8ba" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019c9261-b11f-7c42-9fe4-f094ea09f1ba" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ftnt_ScheduleOfEarningsPerShareTable_019c9261-b11f-7b82-897a-61f6f438a8ba" xlink:to="loc_us-gaap_AwardTypeAxis_019c9261-b11f-7c42-9fe4-f094ea09f1ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-7c42-9fe4-f094ea09f1ba_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019c9261-b11f-7c42-9fe4-f094ea09f1ba" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-7c42-9fe4-f094ea09f1ba_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-7eea-aa2c-031b9cfb3a94" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019c9261-b11f-7c42-9fe4-f094ea09f1ba" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-7eea-aa2c-031b9cfb3a94" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019c9261-b11f-7623-b84a-962e978d9b5d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-7eea-aa2c-031b9cfb3a94" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019c9261-b11f-7623-b84a-962e978d9b5d" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019c9261-b11f-7704-b5c4-dcd036209ec5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-7eea-aa2c-031b9cfb3a94" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019c9261-b11f-7704-b5c4-dcd036209ec5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019c9261-b11f-76e2-bc7a-89237fd19517" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-7eea-aa2c-031b9cfb3a94" xlink:to="loc_us-gaap_PerformanceSharesMember_019c9261-b11f-76e2-bc7a-89237fd19517" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019c9261-b11f-7300-9b12-97b59d08e0d5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_019c9261-b11f-7a65-8870-1adc0c580ba5" xlink:to="loc_us-gaap_NetIncomeLoss_019c9261-b11f-7300-9b12-97b59d08e0d5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasicAbstract_019c9261-b11f-7570-93d5-a426ec6d023b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasicAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_019c9261-b11f-7a65-8870-1adc0c580ba5" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasicAbstract_019c9261-b11f-7570-93d5-a426ec6d023b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_019c9261-b11f-70da-abe5-5593639d88a6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasicAbstract_019c9261-b11f-7570-93d5-a426ec6d023b" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_019c9261-b11f-70da-abe5-5593639d88a6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_019c9261-b11f-7524-bf8c-dd635ba6f793" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_019c9261-b11f-7a65-8870-1adc0c580ba5" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_019c9261-b11f-7524-bf8c-dd635ba6f793" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_019c9261-b11f-777c-9ba9-2b022b572cfa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_019c9261-b11f-76bc-875e-638a36c302bc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_019c9261-b11f-7a65-8870-1adc0c580ba5" xlink:to="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_019c9261-b11f-76bc-875e-638a36c302bc" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_019c9261-b11f-732d-a3c3-0153d1012402" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_019c9261-b11f-76bc-875e-638a36c302bc" xlink:to="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_019c9261-b11f-732d-a3c3-0153d1012402" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_019c9261-b11f-715f-8c0b-0c1a09b7946e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_019c9261-b11f-7a65-8870-1adc0c580ba5" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_019c9261-b11f-715f-8c0b-0c1a09b7946e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareReconciliationAbstract_019c9261-b11f-7da2-b2fb-69a49e4b2026" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareReconciliationAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_019c9261-b11f-7a65-8870-1adc0c580ba5" xlink:to="loc_us-gaap_EarningsPerShareReconciliationAbstract_019c9261-b11f-7da2-b2fb-69a49e4b2026" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareBasic_019c9261-b11f-7808-9741-ffc50ef08c5b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareBasic"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareReconciliationAbstract_019c9261-b11f-7da2-b2fb-69a49e4b2026" xlink:to="loc_us-gaap_EarningsPerShareBasic_019c9261-b11f-7808-9741-ffc50ef08c5b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EarningsPerShareDiluted_019c9261-b11f-7959-95f7-6bce6b4fd0cf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EarningsPerShareDiluted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareReconciliationAbstract_019c9261-b11f-7da2-b2fb-69a49e4b2026" xlink:to="loc_us-gaap_EarningsPerShareDiluted_019c9261-b11f-7959-95f7-6bce6b4fd0cf" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/NetIncomePerShareAntiDilutiveSecuritiesDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#NetIncomePerShareAntiDilutiveSecuritiesDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/NetIncomePerShareAntiDilutiveSecuritiesDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019c9261-b11f-75d4-b923-111edf7156f5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019c9261-b11f-765e-8e48-57702deb2228" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019c9261-b11f-75d4-b923-111edf7156f5" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019c9261-b11f-765e-8e48-57702deb2228" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019c9261-b11f-7564-a5ae-393eb5f15874" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019c9261-b11f-765e-8e48-57702deb2228" xlink:to="loc_us-gaap_AwardTypeAxis_019c9261-b11f-7564-a5ae-393eb5f15874" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-7564-a5ae-393eb5f15874_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019c9261-b11f-7564-a5ae-393eb5f15874" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-7564-a5ae-393eb5f15874_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-77fa-84c6-4a5e2746d47b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019c9261-b11f-7564-a5ae-393eb5f15874" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-77fa-84c6-4a5e2746d47b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019c9261-b11f-7e18-8540-558399c65bf4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-77fa-84c6-4a5e2746d47b" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019c9261-b11f-7e18-8540-558399c65bf4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019c9261-b11f-74cc-a3e8-8a6ee2432c4c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-77fa-84c6-4a5e2746d47b" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019c9261-b11f-74cc-a3e8-8a6ee2432c4c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019c9261-b11f-7abd-ae86-767c7ea7eb12" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-77fa-84c6-4a5e2746d47b" xlink:to="loc_us-gaap_PerformanceSharesMember_019c9261-b11f-7abd-ae86-767c7ea7eb12" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019c9261-b11f-7e62-8968-26da61b99204" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_019c9261-b11f-765e-8e48-57702deb2228" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019c9261-b11f-7e62-8968-26da61b99204" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019c9261-b11f-7e62-8968-26da61b99204_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019c9261-b11f-7e62-8968-26da61b99204" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019c9261-b11f-7e62-8968-26da61b99204_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019c9261-b11f-7a63-b07a-67ceff347ed6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_019c9261-b11f-7e62-8968-26da61b99204" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019c9261-b11f-7a63-b07a-67ceff347ed6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockCompensationPlanMember_019c9261-b11f-7a0c-a00c-46f2a656f892" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockCompensationPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_019c9261-b11f-7a63-b07a-67ceff347ed6" xlink:to="loc_us-gaap_StockCompensationPlanMember_019c9261-b11f-7a0c-a00c-46f2a656f892" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019c9261-b11f-7786-9ab2-64f0c15b8799" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_019c9261-b11f-75d4-b923-111edf7156f5" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_019c9261-b11f-7786-9ab2-64f0c15b8799" xlink:type="arc" order="0"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/DebtNarrativeDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#DebtNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/DebtNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019c9261-b11f-7f43-8c67-65dbe004a13e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019c9261-b11f-71af-b8a8-b7c93872e4f4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c9261-b11f-7f43-8c67-65dbe004a13e" xlink:to="loc_us-gaap_DebtInstrumentTable_019c9261-b11f-71af-b8a8-b7c93872e4f4" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019c9261-b11f-7afe-a6ec-0419ccfa7259" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019c9261-b11f-71af-b8a8-b7c93872e4f4" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019c9261-b11f-7afe-a6ec-0419ccfa7259" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019c9261-b11f-7afe-a6ec-0419ccfa7259_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019c9261-b11f-7afe-a6ec-0419ccfa7259" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019c9261-b11f-7afe-a6ec-0419ccfa7259_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019c9261-b11f-7382-b7b6-83e09ad1417f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019c9261-b11f-7afe-a6ec-0419ccfa7259" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019c9261-b11f-7382-b7b6-83e09ad1417f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorNotesMember_019c9261-b11f-7ff2-8b6b-47cf25b22a37" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019c9261-b11f-7382-b7b6-83e09ad1417f" xlink:to="loc_us-gaap_SeniorNotesMember_019c9261-b11f-7ff2-8b6b-47cf25b22a37" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019c9261-b11f-7837-8368-5c26168a1fd2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019c9261-b11f-71af-b8a8-b7c93872e4f4" xlink:to="loc_us-gaap_DebtInstrumentAxis_019c9261-b11f-7837-8368-5c26168a1fd2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019c9261-b11f-7837-8368-5c26168a1fd2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019c9261-b11f-7837-8368-5c26168a1fd2" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019c9261-b11f-7837-8368-5c26168a1fd2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019c9261-b11f-73f9-9d31-2c87c1d807ce" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019c9261-b11f-7837-8368-5c26168a1fd2" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019c9261-b11f-73f9-9d31-2c87c1d807ce" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_A2026SeniorNotesMember_019c9261-b11f-7ccc-87d6-f433d82936e2" xlink:href="ftnt-20260331.xsd#ftnt_A2026SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019c9261-b11f-73f9-9d31-2c87c1d807ce" xlink:to="loc_ftnt_A2026SeniorNotesMember_019c9261-b11f-7ccc-87d6-f433d82936e2" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_A2031SeniorNotesMember_019c9261-b11f-7996-b642-9b4a1311c52a" xlink:href="ftnt-20260331.xsd#ftnt_A2031SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019c9261-b11f-73f9-9d31-2c87c1d807ce" xlink:to="loc_ftnt_A2031SeniorNotesMember_019c9261-b11f-7996-b642-9b4a1311c52a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019c9261-b11f-73d3-8784-2692b3e87949" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019c9261-b11f-71af-b8a8-b7c93872e4f4" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019c9261-b11f-73d3-8784-2692b3e87949" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c9261-b11f-73d3-8784-2692b3e87949_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019c9261-b11f-73d3-8784-2692b3e87949" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c9261-b11f-73d3-8784-2692b3e87949_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c9261-b11f-71b5-b3ea-86052f73359a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_019c9261-b11f-73d3-8784-2692b3e87949" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c9261-b11f-71b5-b3ea-86052f73359a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_019c9261-b11f-78de-991c-e75d40c9d4f7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_FairValueInputsLevel2Member"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_019c9261-b11f-71b5-b3ea-86052f73359a" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_019c9261-b11f-78de-991c-e75d40c9d4f7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentFaceAmount_019c9261-b11f-700d-b171-6cce64b728c1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentFaceAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c9261-b11f-7f43-8c67-65dbe004a13e" xlink:to="loc_us-gaap_DebtInstrumentFaceAmount_019c9261-b11f-700d-b171-6cce64b728c1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019c9261-b11f-7799-bd7d-3aaae12acb3e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c9261-b11f-7f43-8c67-65dbe004a13e" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019c9261-b11f-7799-bd7d-3aaae12acb3e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RepaymentsOfSeniorDebt_019dd005-89d2-73e3-8eb5-5469c9c626ee" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RepaymentsOfSeniorDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c9261-b11f-7f43-8c67-65dbe004a13e" xlink:to="loc_us-gaap_RepaymentsOfSeniorDebt_019dd005-89d2-73e3-8eb5-5469c9c626ee" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestPayableCurrentAndNoncurrent_019c9261-b11f-72a8-b733-85e63c7c87eb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestPayableCurrentAndNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c9261-b11f-7f43-8c67-65dbe004a13e" xlink:to="loc_us-gaap_InterestPayableCurrentAndNoncurrent_019c9261-b11f-72a8-b733-85e63c7c87eb" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestExpenseDebt_019c9261-b11f-78fa-b706-7738c75858d0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestExpenseDebt"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c9261-b11f-7f43-8c67-65dbe004a13e" xlink:to="loc_us-gaap_InterestExpenseDebt_019c9261-b11f-78fa-b706-7738c75858d0" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InterestCostsCapitalized_019c9261-b11f-775b-a98c-b49ad9e132ab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InterestCostsCapitalized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c9261-b11f-7f43-8c67-65dbe004a13e" xlink:to="loc_us-gaap_InterestCostsCapitalized_019c9261-b11f-775b-a98c-b49ad9e132ab" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtFairValue_019c9261-b11f-71ba-a4a6-28a848ac7df9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c9261-b11f-7f43-8c67-65dbe004a13e" xlink:to="loc_us-gaap_LongTermDebtFairValue_019c9261-b11f-71ba-a4a6-28a848ac7df9" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/DebtSummaryofOutstandingDebtDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#DebtSummaryofOutstandingDebtDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/DebtSummaryofOutstandingDebtDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentLineItems_019c9261-b11f-72ad-96b4-9c5f4642db83" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentTable_019c9261-b11f-76e9-8b9e-2e2cf41d9652" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c9261-b11f-72ad-96b4-9c5f4642db83" xlink:to="loc_us-gaap_DebtInstrumentTable_019c9261-b11f-76e9-8b9e-2e2cf41d9652" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeAxis_019c9261-b11f-7d30-9fec-478b749b8b8c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019c9261-b11f-76e9-8b9e-2e2cf41d9652" xlink:to="loc_us-gaap_LongtermDebtTypeAxis_019c9261-b11f-7d30-9fec-478b749b8b8c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019c9261-b11f-7d30-9fec-478b749b8b8c_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019c9261-b11f-7d30-9fec-478b749b8b8c" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019c9261-b11f-7d30-9fec-478b749b8b8c_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongtermDebtTypeDomain_019c9261-b11f-7cde-855f-682402e3ff80" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongtermDebtTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_LongtermDebtTypeAxis_019c9261-b11f-7d30-9fec-478b749b8b8c" xlink:to="loc_us-gaap_LongtermDebtTypeDomain_019c9261-b11f-7cde-855f-682402e3ff80" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SeniorNotesMember_019c9261-b11f-7f9d-b7f7-ed73e7465be7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LongtermDebtTypeDomain_019c9261-b11f-7cde-855f-682402e3ff80" xlink:to="loc_us-gaap_SeniorNotesMember_019c9261-b11f-7f9d-b7f7-ed73e7465be7" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentAxis_019c9261-b11f-79a7-be56-93c28a0e960d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DebtInstrumentTable_019c9261-b11f-76e9-8b9e-2e2cf41d9652" xlink:to="loc_us-gaap_DebtInstrumentAxis_019c9261-b11f-79a7-be56-93c28a0e960d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019c9261-b11f-79a7-be56-93c28a0e960d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_DebtInstrumentAxis_019c9261-b11f-79a7-be56-93c28a0e960d" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019c9261-b11f-79a7-be56-93c28a0e960d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentNameDomain_019c9261-b11f-7fb5-a446-04badc247201" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_DebtInstrumentAxis_019c9261-b11f-79a7-be56-93c28a0e960d" xlink:to="loc_us-gaap_DebtInstrumentNameDomain_019c9261-b11f-7fb5-a446-04badc247201" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_A2026SeniorNotesMember_019c9261-b11f-788f-bfe5-eee568b9cbab" xlink:href="ftnt-20260331.xsd#ftnt_A2026SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019c9261-b11f-7fb5-a446-04badc247201" xlink:to="loc_ftnt_A2026SeniorNotesMember_019c9261-b11f-788f-bfe5-eee568b9cbab" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_A2031SeniorNotesMember_019c9261-b11f-7c67-b986-d3dd3b87a1ba" xlink:href="ftnt-20260331.xsd#ftnt_A2031SeniorNotesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentNameDomain_019c9261-b11f-7fb5-a446-04badc247201" xlink:to="loc_ftnt_A2031SeniorNotesMember_019c9261-b11f-7c67-b986-d3dd3b87a1ba" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019c9261-b11f-7af3-9e76-df07d69ea871" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateStatedPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c9261-b11f-72ad-96b4-9c5f4642db83" xlink:to="loc_us-gaap_DebtInstrumentInterestRateStatedPercentage_019c9261-b11f-7af3-9e76-df07d69ea871" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019c9261-b11f-7559-af40-500a1f8833b9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentInterestRateEffectivePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c9261-b11f-72ad-96b4-9c5f4642db83" xlink:to="loc_us-gaap_DebtInstrumentInterestRateEffectivePercentage_019c9261-b11f-7559-af40-500a1f8833b9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentCarryingAmount_019c9261-b11f-7763-9c69-1cd95bbe8e28" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentCarryingAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c9261-b11f-72ad-96b4-9c5f4642db83" xlink:to="loc_us-gaap_DebtInstrumentCarryingAmount_019c9261-b11f-7763-9c69-1cd95bbe8e28" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_019c9261-b11f-7317-a11e-882656d9c206" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c9261-b11f-72ad-96b4-9c5f4642db83" xlink:to="loc_us-gaap_DebtInstrumentUnamortizedDiscountPremiumAndDebtIssuanceCostsNet_019c9261-b11f-7317-a11e-882656d9c206" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtCurrent_019c9261-b11f-74db-8d68-1e62e9f154c6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c9261-b11f-72ad-96b4-9c5f4642db83" xlink:to="loc_us-gaap_LongTermDebtCurrent_019c9261-b11f-74db-8d68-1e62e9f154c6" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LongTermDebtNoncurrent_019c9261-b11f-7aad-aca0-1933a6156c5a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LongTermDebtNoncurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DebtInstrumentLineItems_019c9261-b11f-72ad-96b4-9c5f4642db83" xlink:to="loc_us-gaap_LongTermDebtNoncurrent_019c9261-b11f-7aad-aca0-1933a6156c5a" xlink:type="arc" order="5"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/CommitmentsandContingenciesNarrativeDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#CommitmentsandContingenciesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/CommitmentsandContingenciesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesLineItems_019c9261-b11f-74c1-b939-ac73308aaaca" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingenciesLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingenciesTable_019c9261-b11f-7822-9476-d5eb985ee45e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingenciesTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LossContingenciesLineItems_019c9261-b11f-74c1-b939-ac73308aaaca" xlink:to="loc_us-gaap_LossContingenciesTable_019c9261-b11f-7822-9476-d5eb985ee45e" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseAxis_019c9261-b11f-7db5-9f43-34d3f81479d2" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_LitigationCaseAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LossContingenciesTable_019c9261-b11f-7822-9476-d5eb985ee45e" xlink:to="loc_srt_LitigationCaseAxis_019c9261-b11f-7db5-9f43-34d3f81479d2" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_019c9261-b11f-7db5-9f43-34d3f81479d2_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_LitigationCaseAxis_019c9261-b11f-7db5-9f43-34d3f81479d2" xlink:to="loc_srt_LitigationCaseTypeDomain_019c9261-b11f-7db5-9f43-34d3f81479d2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LitigationCaseTypeDomain_019c9261-b11f-7b44-bc30-dc4314b7f7ca" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_LitigationCaseTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_LitigationCaseAxis_019c9261-b11f-7db5-9f43-34d3f81479d2" xlink:to="loc_srt_LitigationCaseTypeDomain_019c9261-b11f-7b44-bc30-dc4314b7f7ca" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_OklahomaFirefightersPensionAndRetirementSystemV.FortinetInc.EtAlAndStateOfRhodeIslandOfficeOfTheGeneralTreasurerV.FortinetInc.EtAlMember_019c9261-b11f-728c-b5f7-6d98c976b195" xlink:href="ftnt-20260331.xsd#ftnt_OklahomaFirefightersPensionAndRetirementSystemV.FortinetInc.EtAlAndStateOfRhodeIslandOfficeOfTheGeneralTreasurerV.FortinetInc.EtAlMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_019c9261-b11f-7b44-bc30-dc4314b7f7ca" xlink:to="loc_ftnt_OklahomaFirefightersPensionAndRetirementSystemV.FortinetInc.EtAlAndStateOfRhodeIslandOfficeOfTheGeneralTreasurerV.FortinetInc.EtAlMember_019c9261-b11f-728c-b5f7-6d98c976b195" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_AloricaV.FortinetMember_019c9261-b11f-7ce0-999a-c51ed4ede8e9" xlink:href="ftnt-20260331.xsd#ftnt_AloricaV.FortinetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_LitigationCaseTypeDomain_019c9261-b11f-7b44-bc30-dc4314b7f7ca" xlink:to="loc_ftnt_AloricaV.FortinetMember_019c9261-b11f-7ce0-999a-c51ed4ede8e9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_InventoryPurchaseObligation_019c9261-b11f-7125-9cc9-5487eefd2589" xlink:href="ftnt-20260331.xsd#ftnt_InventoryPurchaseObligation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019c9261-b11f-74c1-b939-ac73308aaaca" xlink:to="loc_ftnt_InventoryPurchaseObligation_019c9261-b11f-7125-9cc9-5487eefd2589" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_AccruedInventoryPurchaseCommitmentCurrent_019c9261-b11f-7191-a39d-4ffa712ac963" xlink:href="ftnt-20260331.xsd#ftnt_AccruedInventoryPurchaseCommitmentCurrent"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019c9261-b11f-74c1-b939-ac73308aaaca" xlink:to="loc_ftnt_AccruedInventoryPurchaseCommitmentCurrent_019c9261-b11f-7191-a39d-4ffa712ac963" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_InventoryFirmPurchaseCommitmentLoss_019c9261-b11f-7360-bc4e-4d1b33b4d3dc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_InventoryFirmPurchaseCommitmentLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019c9261-b11f-74c1-b939-ac73308aaaca" xlink:to="loc_us-gaap_InventoryFirmPurchaseCommitmentLoss_019c9261-b11f-7360-bc4e-4d1b33b4d3dc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OtherCommitment_019c9261-b11f-7dfa-95ab-e0f9c8678266" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OtherCommitment"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019c9261-b11f-74c1-b939-ac73308aaaca" xlink:to="loc_us-gaap_OtherCommitment_019c9261-b11f-7dfa-95ab-e0f9c8678266" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ContractTermRemainingNonCancelable_019c9261-b11f-71e7-a1e3-0209d2547452" xlink:href="ftnt-20260331.xsd#ftnt_ContractTermRemainingNonCancelable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019c9261-b11f-74c1-b939-ac73308aaaca" xlink:to="loc_ftnt_ContractTermRemainingNonCancelable_019c9261-b11f-71e7-a1e3-0209d2547452" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_OperatingLeaseLiability_019c9261-b11f-760e-aebc-7b0718ade4b7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_OperatingLeaseLiability"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019c9261-b11f-74c1-b939-ac73308aaaca" xlink:to="loc_us-gaap_OperatingLeaseLiability_019c9261-b11f-760e-aebc-7b0718ade4b7" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_LossContingencyAccrualAtCarryingValue_019c9261-b11f-7784-b4a7-63de8b8df011" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_LossContingencyAccrualAtCarryingValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LossContingenciesLineItems_019c9261-b11f-74c1-b939-ac73308aaaca" xlink:to="loc_us-gaap_LossContingencyAccrualAtCarryingValue_019c9261-b11f-7784-b4a7-63de8b8df011" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/EquityPlansandShareRepurchaseProgramRestrictedStockUnitsActivityDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#EquityPlansandShareRepurchaseProgramRestrictedStockUnitsActivityDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/EquityPlansandShareRepurchaseProgramRestrictedStockUnitsActivityDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7c34-9e90-14390b46acf8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019c9261-b11f-71a5-b252-be871366987b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7c34-9e90-14390b46acf8" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019c9261-b11f-71a5-b252-be871366987b" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019c9261-b11f-7abc-bb76-a25a3a90bedb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019c9261-b11f-71a5-b252-be871366987b" xlink:to="loc_us-gaap_AwardTypeAxis_019c9261-b11f-7abc-bb76-a25a3a90bedb" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-7abc-bb76-a25a3a90bedb_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019c9261-b11f-7abc-bb76-a25a3a90bedb" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-7abc-bb76-a25a3a90bedb_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-709f-9754-3d3aa54d44c5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019c9261-b11f-7abc-bb76-a25a3a90bedb" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-709f-9754-3d3aa54d44c5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockCompensationPlanMember_019c9261-b11f-7cc7-903a-acf85f438911" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockCompensationPlanMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-709f-9754-3d3aa54d44c5" xlink:to="loc_us-gaap_StockCompensationPlanMember_019c9261-b11f-7cc7-903a-acf85f438911" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019c9261-b11f-7cb3-88d5-707fd116cf26" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-709f-9754-3d3aa54d44c5" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019c9261-b11f-7cb3-88d5-707fd116cf26" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_RestrictedStockUnitsAndPerformanceSharesMember_019c9261-b11f-7f91-8b8c-d29aa044cc5f" xlink:href="ftnt-20260331.xsd#ftnt_RestrictedStockUnitsAndPerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-709f-9754-3d3aa54d44c5" xlink:to="loc_ftnt_RestrictedStockUnitsAndPerformanceSharesMember_019c9261-b11f-7f91-8b8c-d29aa044cc5f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_019c9261-b11f-7138-bedc-3ada864dd9ce" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7c34-9e90-14390b46acf8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_019c9261-b11f-7138-bedc-3ada864dd9ce" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019c9261-b11f-7d78-b1f7-ebb42be35470" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7c34-9e90-14390b46acf8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019c9261-b11f-7d78-b1f7-ebb42be35470" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019c9261-b11f-7239-927c-365d269395aa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019c9261-b11f-7d78-b1f7-ebb42be35470" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019c9261-b11f-7239-927c-365d269395aa" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019c9261-b11f-7f42-9723-da5e1b8d4ef7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019c9261-b11f-7d78-b1f7-ebb42be35470" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019c9261-b11f-7f42-9723-da5e1b8d4ef7" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019c9261-b11f-78e3-ab31-f26e4beb62ae" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019c9261-b11f-7d78-b1f7-ebb42be35470" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019c9261-b11f-78e3-ab31-f26e4beb62ae" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019c9261-b11f-7fd8-a424-e14fe1d137aa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_019c9261-b11f-7d78-b1f7-ebb42be35470" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019c9261-b11f-7fd8-a424-e14fe1d137aa" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_019c9261-b11f-791d-b3d8-fff4d7be3aca" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019c9261-b11f-72ab-91fc-08d54d542bbc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7c34-9e90-14390b46acf8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019c9261-b11f-72ab-91fc-08d54d542bbc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019c9261-b11f-793d-b5fc-88596d02396a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019c9261-b11f-72ab-91fc-08d54d542bbc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019c9261-b11f-793d-b5fc-88596d02396a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019c9261-b11f-758e-8179-3165bdc02eeb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019c9261-b11f-72ab-91fc-08d54d542bbc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019c9261-b11f-758e-8179-3165bdc02eeb" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019c9261-b11f-789d-b11e-437fb3af420a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019c9261-b11f-72ab-91fc-08d54d542bbc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_019c9261-b11f-789d-b11e-437fb3af420a" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019c9261-b11f-7646-b624-c46d23241b7b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_019c9261-b11f-72ab-91fc-08d54d542bbc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_019c9261-b11f-7646-b624-c46d23241b7b" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_019c9261-b11f-7215-b718-a53a998e9ba3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019dd0d5-576f-7459-87f7-5751563a498f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7c34-9e90-14390b46acf8" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019dd0d5-576f-7459-87f7-5751563a498f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_019c9261-b11f-7381-a0a1-d9f0bd8c546e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7c34-9e90-14390b46acf8" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_019c9261-b11f-7381-a0a1-d9f0bd8c546e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019c9261-b11f-7ce3-aa8f-b9b96a4613a7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7c34-9e90-14390b46acf8" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019c9261-b11f-7ce3-aa8f-b9b96a4613a7" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_019c9261-b11f-78b0-a7a7-e96fe31e7a2b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7c34-9e90-14390b46acf8" xlink:to="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_019c9261-b11f-78b0-a7a7-e96fe31e7a2b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_019c9261-b11f-7611-b8f1-07e51ac5fbed" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7c34-9e90-14390b46acf8" xlink:to="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_019c9261-b11f-7611-b8f1-07e51ac5fbed" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/EquityPlansandShareRepurchaseProgramMarketBasedPSUsDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#EquityPlansandShareRepurchaseProgramMarketBasedPSUsDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/EquityPlansandShareRepurchaseProgramMarketBasedPSUsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-78bb-b9bf-a4c9ed680cf4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019c9261-b11f-75bb-9eb8-309649de462f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-78bb-b9bf-a4c9ed680cf4" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019c9261-b11f-75bb-9eb8-309649de462f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualAxis_019dd0c5-c985-7ae8-864e-b74dad5b6190" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019c9261-b11f-75bb-9eb8-309649de462f" xlink:to="loc_srt_TitleOfIndividualAxis_019dd0c5-c985-7ae8-864e-b74dad5b6190" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019dd0c5-c985-7ae8-864e-b74dad5b6190_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_TitleOfIndividualAxis_019dd0c5-c985-7ae8-864e-b74dad5b6190" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019dd0c5-c985-7ae8-864e-b74dad5b6190_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019dd0c5-c985-7438-b9a6-07e01a0e82cf" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_TitleOfIndividualWithRelationshipToEntityDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_TitleOfIndividualAxis_019dd0c5-c985-7ae8-864e-b74dad5b6190" xlink:to="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019dd0c5-c985-7438-b9a6-07e01a0e82cf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ExecutiveOfficerMember_019dd0c6-4bcf-79ec-9958-4d913b0b0bb4" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ExecutiveOfficerMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_TitleOfIndividualWithRelationshipToEntityDomain_019dd0c5-c985-7438-b9a6-07e01a0e82cf" xlink:to="loc_srt_ExecutiveOfficerMember_019dd0c6-4bcf-79ec-9958-4d913b0b0bb4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019c9261-b11f-75e4-900a-ec94ee4089f4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019c9261-b11f-75bb-9eb8-309649de462f" xlink:to="loc_us-gaap_AwardTypeAxis_019c9261-b11f-75e4-900a-ec94ee4089f4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-75e4-900a-ec94ee4089f4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019c9261-b11f-75e4-900a-ec94ee4089f4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-75e4-900a-ec94ee4089f4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-7f81-a188-21cd70b9c638" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019c9261-b11f-75e4-900a-ec94ee4089f4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-7f81-a188-21cd70b9c638" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019c9261-b11f-7d63-81ee-01d7d5e68cf0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-7f81-a188-21cd70b9c638" xlink:to="loc_us-gaap_PerformanceSharesMember_019c9261-b11f-7d63-81ee-01d7d5e68cf0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019c9261-b11f-7afc-b7d1-093c0fef4996" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019c9261-b11f-75bb-9eb8-309649de462f" xlink:to="loc_srt_RangeAxis_019c9261-b11f-7afc-b7d1-093c0fef4996" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019c9261-b11f-7afc-b7d1-093c0fef4996_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019c9261-b11f-7afc-b7d1-093c0fef4996" xlink:to="loc_srt_RangeMember_019c9261-b11f-7afc-b7d1-093c0fef4996_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019c9261-b11f-7730-a0e0-ad7860a90ed5" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019c9261-b11f-7afc-b7d1-093c0fef4996" xlink:to="loc_srt_RangeMember_019c9261-b11f-7730-a0e0-ad7860a90ed5" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019c9261-b11f-75ac-8252-db7deefccd50" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019c9261-b11f-7730-a0e0-ad7860a90ed5" xlink:to="loc_srt_MinimumMember_019c9261-b11f-75ac-8252-db7deefccd50" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019c9261-b11f-718e-9705-93521f5d512b" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019c9261-b11f-7730-a0e0-ad7860a90ed5" xlink:to="loc_srt_MaximumMember_019c9261-b11f-718e-9705-93521f5d512b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingAxis_019c9261-b11f-7319-b854-016c96c0b9f2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019c9261-b11f-75bb-9eb8-309649de462f" xlink:to="loc_us-gaap_VestingAxis_019c9261-b11f-7319-b854-016c96c0b9f2" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_019c9261-b11f-7319-b854-016c96c0b9f2_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_VestingAxis_019c9261-b11f-7319-b854-016c96c0b9f2" xlink:to="loc_us-gaap_VestingDomain_019c9261-b11f-7319-b854-016c96c0b9f2_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_VestingDomain_019c9261-b11f-753f-9e25-aa58ec622b40" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_VestingDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_VestingAxis_019c9261-b11f-7319-b854-016c96c0b9f2" xlink:to="loc_us-gaap_VestingDomain_019c9261-b11f-753f-9e25-aa58ec622b40" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_019c9261-b11f-78ed-a93a-ab31b10fa2c1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationAwardTrancheOneMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_019c9261-b11f-753f-9e25-aa58ec622b40" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheOneMember_019c9261-b11f-78ed-a93a-ab31b10fa2c1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_019c9261-b11f-736c-b220-70d61e96b441" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationAwardTrancheTwoMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_019c9261-b11f-753f-9e25-aa58ec622b40" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheTwoMember_019c9261-b11f-736c-b220-70d61e96b441" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationAwardTrancheThreeMember_019c9261-b11f-7459-8b35-4e45ccf65cd2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationAwardTrancheThreeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_019c9261-b11f-753f-9e25-aa58ec622b40" xlink:to="loc_us-gaap_ShareBasedCompensationAwardTrancheThreeMember_019c9261-b11f-7459-8b35-4e45ccf65cd2" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ShareBasedPaymentArrangementTrancheFourMember_019c9261-b11f-7a7d-8425-fb0e8e0468c4" xlink:href="ftnt-20260331.xsd#ftnt_ShareBasedPaymentArrangementTrancheFourMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_VestingDomain_019c9261-b11f-753f-9e25-aa58ec622b40" xlink:to="loc_ftnt_ShareBasedPaymentArrangementTrancheFourMember_019c9261-b11f-7a7d-8425-fb0e8e0468c4" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardVariableRangePercentage_019c9261-b11f-7b77-9be3-76fd38affa55" xlink:href="ftnt-20260331.xsd#ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardVariableRangePercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-78bb-b9bf-a4c9ed680cf4" xlink:to="loc_ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardVariableRangePercentage_019c9261-b11f-7b77-9be3-76fd38affa55" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformancePeriod_019c9261-b11f-7956-95f9-82bc5724dec1" xlink:href="ftnt-20260331.xsd#ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformancePeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-78bb-b9bf-a4c9ed680cf4" xlink:to="loc_ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardPerformancePeriod_019c9261-b11f-7956-95f9-82bc5724dec1" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_019c9261-b11f-7dde-b7c9-1f0a10c85b3c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-78bb-b9bf-a4c9ed680cf4" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardAwardVestingRightsPercentage_019c9261-b11f-7dde-b7c9-1f0a10c85b3c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019c9261-b11f-7099-b104-78cceaa84b33" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-78bb-b9bf-a4c9ed680cf4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardAwardVestingPeriod1_019c9261-b11f-7099-b104-78cceaa84b33" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019dd0c6-fa6c-7547-8f56-45d57d8eea62" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-78bb-b9bf-a4c9ed680cf4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_019dd0c6-fa6c-7547-8f56-45d57d8eea62" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019dd0c7-a441-75fe-9538-6be8f1592c15" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-78bb-b9bf-a4c9ed680cf4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019dd0c7-a441-75fe-9538-6be8f1592c15" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019dd0cd-4f8e-7d18-93f8-30453b00eaa1" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-78bb-b9bf-a4c9ed680cf4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_019dd0cd-4f8e-7d18-93f8-30453b00eaa1" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019dd0ce-fc85-71b0-8ed3-61e29d157c6a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-78bb-b9bf-a4c9ed680cf4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_019dd0ce-fc85-71b0-8ed3-61e29d157c6a" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_019c9261-b11f-7262-810d-0d7d6195cffa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-78bb-b9bf-a4c9ed680cf4" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_019c9261-b11f-7262-810d-0d7d6195cffa" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019c9261-b11f-7e14-8127-4a5cfdf56cc3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-78bb-b9bf-a4c9ed680cf4" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019c9261-b11f-7e14-8127-4a5cfdf56cc3" xlink:type="arc" order="9"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/EquityPlansandShareRepurchaseProgramFairValueAssumptionsDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#EquityPlansandShareRepurchaseProgramFairValueAssumptionsDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/EquityPlansandShareRepurchaseProgramFairValueAssumptionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019dd027-5405-7ae1-a784-745dc66d0bdc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019dd027-5405-746d-af1a-fbb8c3c901e3" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019dd027-5405-7ae1-a784-745dc66d0bdc" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019dd027-5405-746d-af1a-fbb8c3c901e3" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019dd027-5405-7687-aea9-783b63e2414d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019dd027-5405-746d-af1a-fbb8c3c901e3" xlink:to="loc_us-gaap_AwardTypeAxis_019dd027-5405-7687-aea9-783b63e2414d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019dd027-5405-7687-aea9-783b63e2414d_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019dd027-5405-7687-aea9-783b63e2414d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019dd027-5405-7687-aea9-783b63e2414d_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019dd027-5405-7fc9-8c6c-7806f6585063" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019dd027-5405-7687-aea9-783b63e2414d" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019dd027-5405-7fc9-8c6c-7806f6585063" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019dd027-5405-722f-a56e-5a043183f9c5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019dd027-5405-7fc9-8c6c-7806f6585063" xlink:to="loc_us-gaap_PerformanceSharesMember_019dd027-5405-722f-a56e-5a043183f9c5" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019dd028-413d-7798-aa2e-acc118a29d2d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019dd027-5405-7fc9-8c6c-7806f6585063" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019dd028-413d-7798-aa2e-acc118a29d2d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_019dd027-5405-7e3c-87f1-195627316286" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019dd027-5405-7ae1-a784-745dc66d0bdc" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_019dd027-5405-7e3c-87f1-195627316286" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_019dd027-5405-74ed-8d68-dde89e270a05" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019dd027-5405-7ae1-a784-745dc66d0bdc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_019dd027-5405-74ed-8d68-dde89e270a05" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_019dd027-5405-75f2-b33c-985aab2ab809" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019dd027-5405-7ae1-a784-745dc66d0bdc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_019dd027-5405-75f2-b33c-985aab2ab809" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_019dd027-5405-72d5-9278-be802f61d31d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019dd027-5405-7ae1-a784-745dc66d0bdc" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_019dd027-5405-72d5-9278-be802f61d31d" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/EquityPlansandShareRepurchaseProgramEmployeeStockOptionsDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#EquityPlansandShareRepurchaseProgramEmployeeStockOptionsDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/EquityPlansandShareRepurchaseProgramEmployeeStockOptionsDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7781-82fb-c0ebf2ce0b0b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019c9261-b11f-70f7-95ec-31613a5bcf88" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7781-82fb-c0ebf2ce0b0b" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019c9261-b11f-70f7-95ec-31613a5bcf88" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019c9261-b11f-790e-9691-c0fb9106e892" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019c9261-b11f-70f7-95ec-31613a5bcf88" xlink:to="loc_us-gaap_AwardTypeAxis_019c9261-b11f-790e-9691-c0fb9106e892" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-790e-9691-c0fb9106e892_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019c9261-b11f-790e-9691-c0fb9106e892" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-790e-9691-c0fb9106e892_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-7215-a5a2-a7cad6d5f808" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019c9261-b11f-790e-9691-c0fb9106e892" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-7215-a5a2-a7cad6d5f808" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019c9261-b11f-7bf0-b710-028c733e5c60" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-7215-a5a2-a7cad6d5f808" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019c9261-b11f-7bf0-b710-028c733e5c60" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_019c9261-b11f-74d3-b06a-35097734eec9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7781-82fb-c0ebf2ce0b0b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_019c9261-b11f-74d3-b06a-35097734eec9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019c9261-b11f-7f48-86c0-3a00bb659763" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7781-82fb-c0ebf2ce0b0b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019c9261-b11f-7f48-86c0-3a00bb659763" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019c9261-b11f-7ee7-ad6d-4c473a47cb8d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019c9261-b11f-7f48-86c0-3a00bb659763" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019c9261-b11f-7ee7-ad6d-4c473a47cb8d" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_019c9261-b11f-75b3-a6aa-d68997ebdab6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019c9261-b11f-7f48-86c0-3a00bb659763" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_019c9261-b11f-75b3-a6aa-d68997ebdab6" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_019c9261-b11f-75c5-8341-1689aaa5f5ea" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019c9261-b11f-7f48-86c0-3a00bb659763" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_019c9261-b11f-75c5-8341-1689aaa5f5ea" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019c9261-b11f-7ffc-a000-24bf0a3aad8e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_019c9261-b11f-7f48-86c0-3a00bb659763" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_019c9261-b11f-7ffc-a000-24bf0a3aad8e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_019c9261-b11f-7700-8520-fdbbb305cca0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019c9261-b11f-7e40-89c1-cfb3ffce3950" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7781-82fb-c0ebf2ce0b0b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019c9261-b11f-7e40-89c1-cfb3ffce3950" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_019c9261-b11f-7d60-bf55-73f4b5690d4b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019c9261-b11f-7e40-89c1-cfb3ffce3950" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_019c9261-b11f-7d60-bf55-73f4b5690d4b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_019c9261-b11f-765c-8463-69f3f22b3d3c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019c9261-b11f-7e40-89c1-cfb3ffce3950" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_019c9261-b11f-765c-8463-69f3f22b3d3c" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_019c9261-b11f-77e0-8b71-c31337a4f00e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019c9261-b11f-7e40-89c1-cfb3ffce3950" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_019c9261-b11f-77e0-8b71-c31337a4f00e" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_019c9261-b11f-77a6-8cda-1735efcc0e2c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_019c9261-b11f-7e40-89c1-cfb3ffce3950" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_019c9261-b11f-77a6-8cda-1735efcc0e2c" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_019c9261-b11f-7211-874c-c1a4fce8b70b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_019c9261-b11f-7e47-883c-d0d9fb3484fa" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7781-82fb-c0ebf2ce0b0b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_019c9261-b11f-7e47-883c-d0d9fb3484fa" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_019c9261-b11f-7779-94d2-aadbe15a07c6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_019c9261-b11f-7e47-883c-d0d9fb3484fa" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_019c9261-b11f-7779-94d2-aadbe15a07c6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_019c9261-b11f-7e82-b2a7-fc740d02090f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_019c9261-b11f-7e47-883c-d0d9fb3484fa" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_019c9261-b11f-7e82-b2a7-fc740d02090f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_019c9261-b11f-7a17-aca5-5455ca524581" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_019c9261-b11f-7e47-883c-d0d9fb3484fa" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_019c9261-b11f-7a17-aca5-5455ca524581" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_019c9261-b11f-7f0c-9e7b-bbed306919f9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_019c9261-b11f-7e47-883c-d0d9fb3484fa" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_019c9261-b11f-7f0c-9e7b-bbed306919f9" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_019c9261-b11f-7ff9-b075-3d1815aadf9c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_019c9261-b11f-7e47-883c-d0d9fb3484fa" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_019c9261-b11f-7ff9-b075-3d1815aadf9c" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_019c9261-b11f-7869-9b16-60fa77c36a33" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_019c9261-b11f-7e47-883c-d0d9fb3484fa" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_019c9261-b11f-7869-9b16-60fa77c36a33" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_019c9261-b11f-7fce-94ca-eff82bba8c99" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_019c9261-b11f-7e47-883c-d0d9fb3484fa" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_019c9261-b11f-7fce-94ca-eff82bba8c99" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_019c9261-b11f-7b6b-8355-0d421c72ce36" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_019c9261-b11f-7e47-883c-d0d9fb3484fa" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_019c9261-b11f-7b6b-8355-0d421c72ce36" xlink:type="arc" order="8"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_019c9261-b11f-700d-b1a8-9bde4ea5142f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_019c9261-b11f-7e47-883c-d0d9fb3484fa" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_019c9261-b11f-700d-b1a8-9bde4ea5142f" xlink:type="arc" order="9"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_019c9261-b11f-72c0-8639-3d0c07b7eee5" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_019c9261-b11f-7e47-883c-d0d9fb3484fa" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_019c9261-b11f-72c0-8639-3d0c07b7eee5" xlink:type="arc" order="10"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_019c9261-b11f-76aa-8a58-f14ba165bfa8" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7781-82fb-c0ebf2ce0b0b" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_019c9261-b11f-76aa-8a58-f14ba165bfa8" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019c9261-b11f-7c44-bfdc-05aad8cfa2a0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7781-82fb-c0ebf2ce0b0b" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_019c9261-b11f-7c44-bfdc-05aad8cfa2a0" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019c9261-b11f-7101-9bde-ea57c561651b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7781-82fb-c0ebf2ce0b0b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_019c9261-b11f-7101-9bde-ea57c561651b" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_019c9261-b11f-74ac-bf59-9046c8cbe57e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7781-82fb-c0ebf2ce0b0b" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_019c9261-b11f-74ac-bf59-9046c8cbe57e" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_019c9261-b11f-771c-8431-a0640c730e70" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7781-82fb-c0ebf2ce0b0b" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_019c9261-b11f-771c-8431-a0640c730e70" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/EquityPlansandShareRepurchaseProgramAllocationofStockBasedCompensationExpenseDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#EquityPlansandShareRepurchaseProgramAllocationofStockBasedCompensationExpenseDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/EquityPlansandShareRepurchaseProgramAllocationofStockBasedCompensationExpenseDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019c9261-b11f-75b8-b1ff-f6410a8383b9" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019c9261-b11f-7b32-b916-227ffd40e748" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019c9261-b11f-75b8-b1ff-f6410a8383b9" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019c9261-b11f-7b32-b916-227ffd40e748" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_019c9261-b11f-7020-a41b-20e3648fc3d4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019c9261-b11f-7b32-b916-227ffd40e748" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_019c9261-b11f-7020-a41b-20e3648fc3d4" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_019c9261-b11f-7020-a41b-20e3648fc3d4_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_019c9261-b11f-7020-a41b-20e3648fc3d4" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_019c9261-b11f-7020-a41b-20e3648fc3d4_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_019c9261-b11f-7e35-beee-6f54dedb3291" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeStatementLocationDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_019c9261-b11f-7020-a41b-20e3648fc3d4" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_019c9261-b11f-7e35-beee-6f54dedb3291" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_CostOfGoodsSoldMember_019c9261-b11f-7240-a579-e1a3065c77ce" xlink:href="ftnt-20260331.xsd#ftnt_CostOfGoodsSoldMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_019c9261-b11f-7e35-beee-6f54dedb3291" xlink:to="loc_ftnt_CostOfGoodsSoldMember_019c9261-b11f-7240-a579-e1a3065c77ce" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_CostOfServicesMember_019c9261-b11f-7a44-9cbb-412e932f788e" xlink:href="ftnt-20260331.xsd#ftnt_CostOfServicesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_019c9261-b11f-7e35-beee-6f54dedb3291" xlink:to="loc_ftnt_CostOfServicesMember_019c9261-b11f-7a44-9cbb-412e932f788e" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_019c9261-b11f-7ecd-b2a2-6a126606874b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_019c9261-b11f-7e35-beee-6f54dedb3291" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_019c9261-b11f-7ecd-b2a2-6a126606874b" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SellingAndMarketingExpenseMember_019c9261-b11f-7812-9fc3-2d153c284399" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SellingAndMarketingExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_019c9261-b11f-7e35-beee-6f54dedb3291" xlink:to="loc_us-gaap_SellingAndMarketingExpenseMember_019c9261-b11f-7812-9fc3-2d153c284399" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpenseMember_019c9261-b11f-787e-a48e-5571238e550e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeneralAndAdministrativeExpenseMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_019c9261-b11f-7e35-beee-6f54dedb3291" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpenseMember_019c9261-b11f-787e-a48e-5571238e550e" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AwardTypeAxis_019c9261-b11f-7326-911a-13bca8afd2da" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AwardTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_019c9261-b11f-7b32-b916-227ffd40e748" xlink:to="loc_us-gaap_AwardTypeAxis_019c9261-b11f-7326-911a-13bca8afd2da" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-7326-911a-13bca8afd2da_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_019c9261-b11f-7326-911a-13bca8afd2da" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-7326-911a-13bca8afd2da_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-78b1-a1af-4ac41e6db52f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_019c9261-b11f-7326-911a-13bca8afd2da" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-78b1-a1af-4ac41e6db52f" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_019c9261-b11f-7ccc-bfc4-9bc4014cf059" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RestrictedStockUnitsRSUMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-78b1-a1af-4ac41e6db52f" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_019c9261-b11f-7ccc-bfc4-9bc4014cf059" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeStockOptionMember_019c9261-b11f-7242-ac47-83041137f475" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeStockOptionMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-78b1-a1af-4ac41e6db52f" xlink:to="loc_us-gaap_EmployeeStockOptionMember_019c9261-b11f-7242-ac47-83041137f475" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PerformanceSharesMember_019c9261-b11f-7a60-ae11-a8c4c669f922" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PerformanceSharesMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_019c9261-b11f-78b1-a1af-4ac41e6db52f" xlink:to="loc_us-gaap_PerformanceSharesMember_019c9261-b11f-7a60-ae11-a8c4c669f922" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_019c9261-b11f-7d0d-9b03-79f91429b1eb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AllocatedShareBasedCompensationExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019c9261-b11f-75b8-b1ff-f6410a8383b9" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_019c9261-b11f-7d0d-9b03-79f91429b1eb" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions_019c9261-b11f-74f5-a24d-e07f8fcf2c6b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_019c9261-b11f-75b8-b1ff-f6410a8383b9" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions_019c9261-b11f-74f5-a24d-e07f8fcf2c6b" xlink:type="arc" order="1"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/EquityPlansandShareRepurchaseProgramShareRepurchaseProgramDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#EquityPlansandShareRepurchaseProgramShareRepurchaseProgramDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/EquityPlansandShareRepurchaseProgramShareRepurchaseProgramDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7ece-b648-ea98bdcbcdec" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019c9261-b11f-7fd4-9efc-6c16f15e3b31" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7ece-b648-ea98bdcbcdec" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019c9261-b11f-7fd4-9efc-6c16f15e3b31" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramAxis_019c9261-b11f-711e-a276-585e64a5532b" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ShareRepurchaseProgramAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_019c9261-b11f-7fd4-9efc-6c16f15e3b31" xlink:to="loc_srt_ShareRepurchaseProgramAxis_019c9261-b11f-711e-a276-585e64a5532b" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramDomain_019c9261-b11f-711e-a276-585e64a5532b_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ShareRepurchaseProgramAxis_019c9261-b11f-711e-a276-585e64a5532b" xlink:to="loc_srt_ShareRepurchaseProgramDomain_019c9261-b11f-711e-a276-585e64a5532b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramDomain_019c9261-b11f-78bd-bf27-616cdfeb2b36" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ShareRepurchaseProgramAxis_019c9261-b11f-711e-a276-585e64a5532b" xlink:to="loc_srt_ShareRepurchaseProgramDomain_019c9261-b11f-78bd-bf27-616cdfeb2b36" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ShareRepurchaseProgramMember_019c9261-b11f-7374-9e4f-86c10524d35b" xlink:href="ftnt-20260331.xsd#ftnt_ShareRepurchaseProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramDomain_019c9261-b11f-78bd-bf27-616cdfeb2b36" xlink:to="loc_ftnt_ShareRepurchaseProgramMember_019c9261-b11f-7374-9e4f-86c10524d35b" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_StockRepurchaseProgramAuthorizedAmountApprovedIncrease_019c9261-b11f-7721-b61c-27088f6af5cc" xlink:href="ftnt-20260331.xsd#ftnt_StockRepurchaseProgramAuthorizedAmountApprovedIncrease"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7ece-b648-ea98bdcbcdec" xlink:to="loc_ftnt_StockRepurchaseProgramAuthorizedAmountApprovedIncrease_019c9261-b11f-7721-b61c-27088f6af5cc" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StockRepurchaseProgramAuthorizedAmount1_019c9261-b11f-720e-aeac-9587c969ead6" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StockRepurchaseProgramAuthorizedAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7ece-b648-ea98bdcbcdec" xlink:to="loc_srt_StockRepurchaseProgramAuthorizedAmount1_019c9261-b11f-720e-aeac-9587c969ead6" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_019c9261-b11f-7c6e-8e13-726076ceb60a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7ece-b648-ea98bdcbcdec" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_019c9261-b11f-7c6e-8e13-726076ceb60a" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_019c9261-b11f-7233-8baa-d7305bae3c83" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockAcquiredAverageCostPerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7ece-b648-ea98bdcbcdec" xlink:to="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_019c9261-b11f-7233-8baa-d7305bae3c83" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_019c9261-b11f-733e-a915-e8807461f142" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7ece-b648-ea98bdcbcdec" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_019c9261-b11f-733e-a915-e8807461f142" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ShareRepurchaseProgramExciseTax_019c9261-b11f-795e-bab1-7067fc9bd545" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ShareRepurchaseProgramExciseTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7ece-b648-ea98bdcbcdec" xlink:to="loc_us-gaap_ShareRepurchaseProgramExciseTax_019c9261-b11f-795e-bab1-7067fc9bd545" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_019c9261-b11f-7da4-9053-feaa0ab075ca" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_019c9261-b11f-7ece-b648-ea98bdcbcdec" xlink:to="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_019c9261-b11f-7da4-9053-feaa0ab075ca" xlink:type="arc" order="6"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/IncomeTaxesNarrativeDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#IncomeTaxesNarrativeDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/IncomeTaxesNarrativeDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxContingencyLineItems_019c9261-b11f-78fa-88a2-80e297a04933" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxContingencyLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxContingencyTable_019c9261-b11f-7180-885c-ebf61574b962" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxContingencyTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_019c9261-b11f-78fa-88a2-80e297a04933" xlink:to="loc_us-gaap_IncomeTaxContingencyTable_019c9261-b11f-7180-885c-ebf61574b962" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeAxis_019c9261-b11f-7161-aafa-35af410712cc" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_IncomeTaxContingencyTable_019c9261-b11f-7180-885c-ebf61574b962" xlink:to="loc_srt_RangeAxis_019c9261-b11f-7161-aafa-35af410712cc" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019c9261-b11f-7161-aafa-35af410712cc_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_019c9261-b11f-7161-aafa-35af410712cc" xlink:to="loc_srt_RangeMember_019c9261-b11f-7161-aafa-35af410712cc_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_RangeMember_019c9261-b11f-7295-8c34-643a353b9d13" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_RangeMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_019c9261-b11f-7161-aafa-35af410712cc" xlink:to="loc_srt_RangeMember_019c9261-b11f-7295-8c34-643a353b9d13" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MinimumMember_019c9261-b11f-7ba0-ba85-27257c67cbd1" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MinimumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019c9261-b11f-7295-8c34-643a353b9d13" xlink:to="loc_srt_MinimumMember_019c9261-b11f-7ba0-ba85-27257c67cbd1" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MaximumMember_019c9261-b11f-728b-a1b2-10214d8ae531" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MaximumMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_019c9261-b11f-7295-8c34-643a353b9d13" xlink:to="loc_srt_MaximumMember_019c9261-b11f-728b-a1b2-10214d8ae531" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_019c9261-b11f-7a1e-8517-258754147136" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateContinuingOperations"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_019c9261-b11f-78fa-88a2-80e297a04933" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_019c9261-b11f-7a1e-8517-258754147136" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_EffectiveIncomeTaxRateReconciliationatFederalStatutoryIncomeTaxRateStateandLocalIncomeTaxesandForeignIncomeTaxRateDifferentialAmount_019c9261-b11f-7c24-8844-ec48365cf7ac" xlink:href="ftnt-20260331.xsd#ftnt_EffectiveIncomeTaxRateReconciliationatFederalStatutoryIncomeTaxRateStateandLocalIncomeTaxesandForeignIncomeTaxRateDifferentialAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_019c9261-b11f-78fa-88a2-80e297a04933" xlink:to="loc_ftnt_EffectiveIncomeTaxRateReconciliationatFederalStatutoryIncomeTaxRateStateandLocalIncomeTaxesandForeignIncomeTaxRateDifferentialAmount_019c9261-b11f-7c24-8844-ec48365cf7ac" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_EffectiveIncomeTaxRateReconciliationDerecognitionOfDeferredTaxAssetsAmount_019c9261-b11f-7e93-94cb-509cba3d264d" xlink:href="ftnt-20260331.xsd#ftnt_EffectiveIncomeTaxRateReconciliationDerecognitionOfDeferredTaxAssetsAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_019c9261-b11f-78fa-88a2-80e297a04933" xlink:to="loc_ftnt_EffectiveIncomeTaxRateReconciliationDerecognitionOfDeferredTaxAssetsAmount_019c9261-b11f-7e93-94cb-509cba3d264d" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateReconciliationFdiiAmount_019c9261-b11f-784d-bb4f-429197652ae6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EffectiveIncomeTaxRateReconciliationFdiiAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_019c9261-b11f-78fa-88a2-80e297a04933" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateReconciliationFdiiAmount_019c9261-b11f-784d-bb4f-429197652ae6" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_EffectiveIncomeTaxReconciliationExcessTaxBenefitonStockbasedCompensation_019c9261-b11f-7d29-a190-867a47bcd032" xlink:href="ftnt-20260331.xsd#ftnt_EffectiveIncomeTaxReconciliationExcessTaxBenefitonStockbasedCompensation"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_019c9261-b11f-78fa-88a2-80e297a04933" xlink:to="loc_ftnt_EffectiveIncomeTaxReconciliationExcessTaxBenefitonStockbasedCompensation_019c9261-b11f-7d29-a190-867a47bcd032" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefits_019c9261-b11f-7d01-9a7e-898af685ea26" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnrecognizedTaxBenefits"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_019c9261-b11f-78fa-88a2-80e297a04933" xlink:to="loc_us-gaap_UnrecognizedTaxBenefits_019c9261-b11f-7d01-9a7e-898af685ea26" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate_019c9261-b11f-762e-9be6-95889209dfbe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_019c9261-b11f-78fa-88a2-80e297a04933" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate_019c9261-b11f-762e-9be6-95889209dfbe" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued_019c9261-b11f-7e60-b3ca-ac752819a2ab" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeTaxContingencyLineItems_019c9261-b11f-78fa-88a2-80e297a04933" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsIncomeTaxPenaltiesAndInterestAccrued_019c9261-b11f-7e60-b3ca-ac752819a2ab" xlink:type="arc" order="7"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/SegmentInformationReportableSegmentDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#SegmentInformationReportableSegmentDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/SegmentInformationReportableSegmentDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019c9261-b11f-714f-bed1-bed29f5ea436" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019c9261-b11f-7460-83f6-1e8a140e6ae6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019c9261-b11f-714f-bed1-bed29f5ea436" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019c9261-b11f-7460-83f6-1e8a140e6ae6" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StatementBusinessSegmentsAxis_019c9261-b11f-7131-817a-7cbee87571ad" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StatementBusinessSegmentsAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019c9261-b11f-7460-83f6-1e8a140e6ae6" xlink:to="loc_us-gaap_StatementBusinessSegmentsAxis_019c9261-b11f-7131-817a-7cbee87571ad" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019c9261-b11f-7131-817a-7cbee87571ad_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019c9261-b11f-7131-817a-7cbee87571ad" xlink:to="loc_us-gaap_SegmentDomain_019c9261-b11f-7131-817a-7cbee87571ad_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentDomain_019c9261-b11f-7e3f-bf97-2196c4d32b00" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementBusinessSegmentsAxis_019c9261-b11f-7131-817a-7cbee87571ad" xlink:to="loc_us-gaap_SegmentDomain_019c9261-b11f-7e3f-bf97-2196c4d32b00" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ReportableSegmentMember_019c9261-b11f-75c4-8ad8-83132ca726f9" xlink:href="ftnt-20260331.xsd#ftnt_ReportableSegmentMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentDomain_019c9261-b11f-7e3f-bf97-2196c4d32b00" xlink:to="loc_ftnt_ReportableSegmentMember_019c9261-b11f-75c4-8ad8-83132ca726f9" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductOrServiceAxis_019c9261-b11f-75c2-bc5d-666973ba78fd" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductOrServiceAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019c9261-b11f-7460-83f6-1e8a140e6ae6" xlink:to="loc_srt_ProductOrServiceAxis_019c9261-b11f-75c2-bc5d-666973ba78fd" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019c9261-b11f-75c2-bc5d-666973ba78fd_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_019c9261-b11f-75c2-bc5d-666973ba78fd" xlink:to="loc_srt_ProductsAndServicesDomain_019c9261-b11f-75c2-bc5d-666973ba78fd_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ProductsAndServicesDomain_019c9261-b11f-7b1e-8c7c-ffc329e90ac9" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ProductsAndServicesDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_019c9261-b11f-75c2-bc5d-666973ba78fd" xlink:to="loc_srt_ProductsAndServicesDomain_019c9261-b11f-7b1e-8c7c-ffc329e90ac9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ProductMember_019c9261-b11f-7d44-bfed-496c57d869a4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ProductMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019c9261-b11f-7b1e-8c7c-ffc329e90ac9" xlink:to="loc_us-gaap_ProductMember_019c9261-b11f-7d44-bfed-496c57d869a4" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ServiceMember_019c9261-b11f-7ea8-a835-16549e825dde" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ServiceMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_019c9261-b11f-7b1e-8c7c-ffc329e90ac9" xlink:to="loc_us-gaap_ServiceMember_019c9261-b11f-7ea8-a835-16549e825dde" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019c9261-b11f-7a3b-b03b-bdfa66876095" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019c9261-b11f-714f-bed1-bed29f5ea436" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019c9261-b11f-7a3b-b03b-bdfa66876095" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CostOfGoodsAndServicesSold_019c9261-b11f-7eb8-9e12-7e095f8f84ef" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CostOfGoodsAndServicesSold"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019c9261-b11f-714f-bed1-bed29f5ea436" xlink:to="loc_us-gaap_CostOfGoodsAndServicesSold_019c9261-b11f-7eb8-9e12-7e095f8f84ef" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_019c9261-b11f-7a5a-9fa0-42a28d348abc" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ResearchAndDevelopmentExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019c9261-b11f-714f-bed1-bed29f5ea436" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_019c9261-b11f-7a5a-9fa0-42a28d348abc" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_SellingAndMarketingExpenseAdjusted_019c9261-b11f-79c4-a46c-e2284741f402" xlink:href="ftnt-20260331.xsd#ftnt_SellingAndMarketingExpenseAdjusted"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019c9261-b11f-714f-bed1-bed29f5ea436" xlink:to="loc_ftnt_SellingAndMarketingExpenseAdjusted_019c9261-b11f-79c4-a46c-e2284741f402" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SalesCommissionsAndFees_019c9261-b11f-7a04-98fb-a84f3742cc90" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SalesCommissionsAndFees"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019c9261-b11f-714f-bed1-bed29f5ea436" xlink:to="loc_us-gaap_SalesCommissionsAndFees_019c9261-b11f-7a04-98fb-a84f3742cc90" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpense_019c9261-b11f-7913-8e21-664324b47af4" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_GeneralAndAdministrativeExpense"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019c9261-b11f-714f-bed1-bed29f5ea436" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpense_019c9261-b11f-7913-8e21-664324b47af4" xlink:type="arc" order="5"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingOtherItemAmount_019c9261-b11f-7d69-82d1-a0c6eade60b0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingOtherItemAmount"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019c9261-b11f-714f-bed1-bed29f5ea436" xlink:to="loc_us-gaap_SegmentReportingOtherItemAmount_019c9261-b11f-7d69-82d1-a0c6eade60b0" xlink:type="arc" order="6"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_019c9261-b11f-7a23-b864-aa3e9a783abb" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_IncomeTaxExpenseBenefit"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019c9261-b11f-714f-bed1-bed29f5ea436" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_019c9261-b11f-7a23-b864-aa3e9a783abb" xlink:type="arc" order="7"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_NetIncomeLoss_019c9261-b11f-74f3-8b88-dfe72ddb4128" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_NetIncomeLoss"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019c9261-b11f-714f-bed1-bed29f5ea436" xlink:to="loc_us-gaap_NetIncomeLoss_019c9261-b11f-74f3-8b88-dfe72ddb4128" xlink:type="arc" order="8"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/SegmentInformationGeographicalandConcentrationRiskDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#SegmentInformationGeographicalandConcentrationRiskDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/SegmentInformationGeographicalandConcentrationRiskDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_019c9261-b120-753f-8ebb-8f1c007e5668" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SegmentReportingInformationLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019c9261-b120-7ce0-a2f7-0980bb32fcb2" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019c9261-b120-753f-8ebb-8f1c007e5668" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019c9261-b120-7ce0-a2f7-0980bb32fcb2" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_StatementGeographicalAxis_019c9261-b120-7e71-87b9-46fc248b8aee" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_StatementGeographicalAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019c9261-b120-7ce0-a2f7-0980bb32fcb2" xlink:to="loc_srt_StatementGeographicalAxis_019c9261-b120-7e71-87b9-46fc248b8aee" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019c9261-b120-7e71-87b9-46fc248b8aee_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_019c9261-b120-7e71-87b9-46fc248b8aee" xlink:to="loc_srt_SegmentGeographicalDomain_019c9261-b120-7e71-87b9-46fc248b8aee_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_SegmentGeographicalDomain_019c9261-b120-79af-8c11-80ef324100d9" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_SegmentGeographicalDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_019c9261-b120-7e71-87b9-46fc248b8aee" xlink:to="loc_srt_SegmentGeographicalDomain_019c9261-b120-79af-8c11-80ef324100d9" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_AmericasMember_019c9261-b120-778b-8b78-23401eb09182" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_AmericasMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019c9261-b120-79af-8c11-80ef324100d9" xlink:to="loc_srt_AmericasMember_019c9261-b120-778b-8b78-23401eb09182" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_US_019c9261-b120-7da9-b330-0fdea22306d3" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_US"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_AmericasMember_019c9261-b120-778b-8b78-23401eb09182" xlink:to="loc_country_US_019c9261-b120-7da9-b330-0fdea22306d3" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_country_CA_019c9261-b120-7318-ae38-c8c3900984a5" xlink:href="https://xbrl.sec.gov/country/2025/country-2025.xsd#country_CA"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_AmericasMember_019c9261-b120-778b-8b78-23401eb09182" xlink:to="loc_country_CA_019c9261-b120-7318-ae38-c8c3900984a5" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_OtherAmericasMember_019c9261-b120-7164-a0be-1c9144960fc0" xlink:href="ftnt-20260331.xsd#ftnt_OtherAmericasMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_AmericasMember_019c9261-b120-778b-8b78-23401eb09182" xlink:to="loc_ftnt_OtherAmericasMember_019c9261-b120-7164-a0be-1c9144960fc0" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_LatinAmericaMember_019c9261-b120-7b18-aa1b-4896ef22c3f1" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_LatinAmericaMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_AmericasMember_019c9261-b120-778b-8b78-23401eb09182" xlink:to="loc_srt_LatinAmericaMember_019c9261-b120-7b18-aa1b-4896ef22c3f1" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_EMEAMember_019c9261-b120-7bdf-892f-731f6eef387a" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_EMEAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019c9261-b120-79af-8c11-80ef324100d9" xlink:to="loc_us-gaap_EMEAMember_019c9261-b120-7bdf-892f-731f6eef387a" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_AsiaPacificMember_019c9261-b120-7a90-8d2f-e28a5348b202" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_AsiaPacificMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_019c9261-b120-79af-8c11-80ef324100d9" xlink:to="loc_srt_AsiaPacificMember_019c9261-b120-7a90-8d2f-e28a5348b202" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_MajorCustomersAxis_019c9261-b120-7516-abae-fbe332f49d03" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_MajorCustomersAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019c9261-b120-7ce0-a2f7-0980bb32fcb2" xlink:to="loc_srt_MajorCustomersAxis_019c9261-b120-7516-abae-fbe332f49d03" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_019c9261-b120-7516-abae-fbe332f49d03_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_019c9261-b120-7516-abae-fbe332f49d03" xlink:to="loc_srt_NameOfMajorCustomerDomain_019c9261-b120-7516-abae-fbe332f49d03_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_NameOfMajorCustomerDomain_019c9261-b120-7c5b-88ff-42fab3458850" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_NameOfMajorCustomerDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_019c9261-b120-7516-abae-fbe332f49d03" xlink:to="loc_srt_NameOfMajorCustomerDomain_019c9261-b120-7c5b-88ff-42fab3458850" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_DistributorAMember_019c9261-b120-71a6-9436-8e1505fbc858" xlink:href="ftnt-20260331.xsd#ftnt_DistributorAMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019c9261-b120-7c5b-88ff-42fab3458850" xlink:to="loc_ftnt_DistributorAMember_019c9261-b120-71a6-9436-8e1505fbc858" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_DistributorBMember_019c9261-b120-751f-9440-977a90697879" xlink:href="ftnt-20260331.xsd#ftnt_DistributorBMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019c9261-b120-7c5b-88ff-42fab3458850" xlink:to="loc_ftnt_DistributorBMember_019c9261-b120-751f-9440-977a90697879" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_DistributorCMember_019c9261-b120-748f-bad1-d971996d220f" xlink:href="ftnt-20260331.xsd#ftnt_DistributorCMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_019c9261-b120-7c5b-88ff-42fab3458850" xlink:to="loc_ftnt_DistributorCMember_019c9261-b120-748f-bad1-d971996d220f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_019c9261-b120-7d0c-bfeb-4d72c80d445b" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019c9261-b120-7ce0-a2f7-0980bb32fcb2" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_019c9261-b120-7d0c-bfeb-4d72c80d445b" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019c9261-b120-7d0c-bfeb-4d72c80d445b_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019c9261-b120-7d0c-bfeb-4d72c80d445b" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019c9261-b120-7d0c-bfeb-4d72c80d445b_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_019c9261-b120-7b88-ba9d-dc5dd5ccd8fe" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_019c9261-b120-7d0c-bfeb-4d72c80d445b" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_019c9261-b120-7b88-ba9d-dc5dd5ccd8fe" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_CustomerConcentrationRiskMember_019c9261-b120-769e-8aba-2d88166d8d5e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_CustomerConcentrationRiskMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_019c9261-b120-7b88-ba9d-dc5dd5ccd8fe" xlink:to="loc_us-gaap_CustomerConcentrationRiskMember_019c9261-b120-769e-8aba-2d88166d8d5e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019c9261-b120-75ae-af96-a9c34ddac590" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_019c9261-b120-7ce0-a2f7-0980bb32fcb2" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019c9261-b120-75ae-af96-a9c34ddac590" xlink:type="arc" order="4"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019c9261-b120-75ae-af96-a9c34ddac590_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019c9261-b120-75ae-af96-a9c34ddac590" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019c9261-b120-75ae-af96-a9c34ddac590_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019c9261-b120-77b4-8a4e-c1757ea2b5bf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskBenchmarkDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_019c9261-b120-75ae-af96-a9c34ddac590" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019c9261-b120-77b4-8a4e-c1757ea2b5bf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SalesRevenueNetMember_019c9261-b120-75f2-9ef8-4bff7af110a0" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SalesRevenueNetMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019c9261-b120-77b4-8a4e-c1757ea2b5bf" xlink:to="loc_us-gaap_SalesRevenueNetMember_019c9261-b120-75f2-9ef8-4bff7af110a0" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_AccountsReceivableMember_019c9261-b120-7fd5-963a-9feb36c4e1ea" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_AccountsReceivableMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_019c9261-b120-77b4-8a4e-c1757ea2b5bf" xlink:to="loc_us-gaap_AccountsReceivableMember_019c9261-b120-7fd5-963a-9feb36c4e1ea" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019c9261-b120-71bc-baae-0bd39e2095bf" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019c9261-b120-753f-8ebb-8f1c007e5668" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_019c9261-b120-71bc-baae-0bd39e2095bf" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_019c9261-b120-7747-bd31-4adee5122eb7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_PropertyPlantAndEquipmentNet"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019c9261-b120-753f-8ebb-8f1c007e5668" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_019c9261-b120-7747-bd31-4adee5122eb7" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_019c9261-b120-747f-a2e4-84a96b3dd17d" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_ConcentrationRiskPercentage1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_019c9261-b120-753f-8ebb-8f1c007e5668" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_019c9261-b120-747f-a2e4-84a96b3dd17d" xlink:type="arc" order="2"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://www.fortinet.com/role/SubsequentEventDetails" xlink:type="simple" xlink:href="ftnt-20260331.xsd#SubsequentEventDetails"/>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/SubsequentEventDetails" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventLineItems_019c9261-b120-7919-92be-84f4949ab9d7" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventLineItems"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTable_019c9261-b120-7059-95d5-f223e9337d3f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTable"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_019c9261-b120-7919-92be-84f4949ab9d7" xlink:to="loc_us-gaap_SubsequentEventTable_019c9261-b120-7059-95d5-f223e9337d3f" xlink:type="arc" order="1" xbrldt:closed="true" xbrldt:contextElement="segment"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_019c9261-b120-7ac0-9502-5dc190c9f029" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019c9261-b120-7059-95d5-f223e9337d3f" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_019c9261-b120-7ac0-9502-5dc190c9f029" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019c9261-b120-7ac0-9502-5dc190c9f029_default" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019c9261-b120-7ac0-9502-5dc190c9f029" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019c9261-b120-7ac0-9502-5dc190c9f029_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_019c9261-b120-7244-b63d-b5edb113cd71" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventTypeDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_019c9261-b120-7ac0-9502-5dc190c9f029" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_019c9261-b120-7244-b63d-b5edb113cd71" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_SubsequentEventMember_019c9261-b120-7930-bb2d-72b9502df70e" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_SubsequentEventMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_019c9261-b120-7244-b63d-b5edb113cd71" xlink:to="loc_us-gaap_SubsequentEventMember_019c9261-b120-7930-bb2d-72b9502df70e" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramAxis_019c9261-b120-7aff-9ee1-09d285e63e53" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ShareRepurchaseProgramAxis"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_019c9261-b120-7059-95d5-f223e9337d3f" xlink:to="loc_srt_ShareRepurchaseProgramAxis_019c9261-b120-7aff-9ee1-09d285e63e53" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramDomain_019c9261-b120-7aff-9ee1-09d285e63e53_default" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ShareRepurchaseProgramAxis_019c9261-b120-7aff-9ee1-09d285e63e53" xlink:to="loc_srt_ShareRepurchaseProgramDomain_019c9261-b120-7aff-9ee1-09d285e63e53_default" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_srt_ShareRepurchaseProgramDomain_019c9261-b120-7ca6-ad07-c43b810160cf" xlink:href="https://xbrl.fasb.org/srt/2025/elts/srt-2025.xsd#srt_ShareRepurchaseProgramDomain"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ShareRepurchaseProgramAxis_019c9261-b120-7aff-9ee1-09d285e63e53" xlink:to="loc_srt_ShareRepurchaseProgramDomain_019c9261-b120-7ca6-ad07-c43b810160cf" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_ShareRepurchaseProgramMember_019c9261-b120-7e22-abe2-977b7cae01bd" xlink:href="ftnt-20260331.xsd#ftnt_ShareRepurchaseProgramMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ShareRepurchaseProgramDomain_019c9261-b120-7ca6-ad07-c43b810160cf" xlink:to="loc_ftnt_ShareRepurchaseProgramMember_019c9261-b120-7e22-abe2-977b7cae01bd" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_019c9261-b120-785f-acba-53c5327da8d6" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019c9261-b120-7919-92be-84f4949ab9d7" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_019c9261-b120-785f-acba-53c5327da8d6" xlink:type="arc" order="0"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_019c9261-b120-7e94-ad5d-0809c407529c" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_TreasuryStockAcquiredAverageCostPerShare"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019c9261-b120-7919-92be-84f4949ab9d7" xlink:to="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_019c9261-b120-7e94-ad5d-0809c407529c" xlink:type="arc" order="1"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_019c9261-b120-77c4-8fe3-9ba9e8ce304f" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019c9261-b120-7919-92be-84f4949ab9d7" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_019c9261-b120-77c4-8fe3-9ba9e8ce304f" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_019c9261-b120-7cd0-9e1f-a4dd618ebf04" xlink:href="https://xbrl.fasb.org/us-gaap/2025/elts/us-gaap-2025.xsd#us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_019c9261-b120-7919-92be-84f4949ab9d7" xlink:to="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_019c9261-b120-7cd0-9e1f-a4dd618ebf04" xlink:type="arc" order="3"/>
  </link:definitionLink>
  <link:roleRef roleURI="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="simple" xlink:href="https://xbrl.sec.gov/ecd/2025/ecd-2025.xsd#IndividualsOnly"/>
  <link:definitionLink xlink:role="http://xbrl.sec.gov/ecd/role/IndividualsOnly" xlink:type="extended">
    <link:loc xlink:type="locator" xlink:label="loc_ecd_AllIndividualsMember" xlink:href="https://xbrl.sec.gov/ecd/2025/ecd-2025.xsd#ecd_AllIndividualsMember"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_KenXieMember_019dd04d-145f-7bd3-a42b-48106e605dd4" xlink:href="ftnt-20260331.xsd#ftnt_KenXieMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_ftnt_KenXieMember_019dd04d-145f-7bd3-a42b-48106e605dd4" xlink:type="arc" order="2"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_MichaelXieMember_019dd0bd-049e-77b0-ae63-2a5522502a7f" xlink:href="ftnt-20260331.xsd#ftnt_MichaelXieMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_ftnt_MichaelXieMember_019dd0bd-049e-77b0-ae63-2a5522502a7f" xlink:type="arc" order="3"/>
    <link:loc xlink:type="locator" xlink:label="loc_ftnt_JohnWhittleMember_019c9261-afc4-7e48-9ccc-b3669bf9ebff" xlink:href="ftnt-20260331.xsd#ftnt_JohnWhittleMember"/>
    <link:definitionArc xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ecd_AllIndividualsMember" xlink:to="loc_ftnt_JohnWhittleMember_019c9261-afc4-7e48-9ccc-b3669bf9ebff" xlink:type="arc" order="4"/>
  </link:definitionLink>
</link:linkbase>
