<?xml version="1.0" encoding="US-ASCII"?>
<!--XBRL Document Created with Wdesk from Workiva-->
<!-- -->
<link:linkbase xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" xmlns:link="http://www.xbrl.org/2003/linkbase" xmlns:xbrldt="http://xbrl.org/2005/xbrldt" xmlns:xlink="http://www.w3.org/1999/xlink" xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance">
  <link:roleRef roleURI="http://www.fortinet.com/role/AccumulatedOtherComprehensiveIncomeLoss" xlink:href="ftnt-20190930.xsd#AccumulatedOtherComprehensiveIncomeLoss" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/AccumulatedOtherComprehensiveIncomeLossChangesInAccumulatedBalancesOfOtherComprehensiveLossDetails" xlink:href="ftnt-20190930.xsd#AccumulatedOtherComprehensiveIncomeLossChangesInAccumulatedBalancesOfOtherComprehensiveLossDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/AccumulatedOtherComprehensiveIncomeLossTables" xlink:href="ftnt-20190930.xsd#AccumulatedOtherComprehensiveIncomeLossTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/BusinessCombinations" xlink:href="ftnt-20190930.xsd#BusinessCombinations" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/BusinessCombinationsAdditionalInformationDetails" xlink:href="ftnt-20190930.xsd#BusinessCombinationsAdditionalInformationDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/CommitmentsAndContingencies" xlink:href="ftnt-20190930.xsd#CommitmentsAndContingencies" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/CommitmentsAndContingenciesDetails" xlink:href="ftnt-20190930.xsd#CommitmentsAndContingenciesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/CommitmentsAndContingenciesMinimumOperatingLeasePaymentsDetails" xlink:href="ftnt-20190930.xsd#CommitmentsAndContingenciesMinimumOperatingLeasePaymentsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/CommitmentsAndContingenciesTables" xlink:href="ftnt-20190930.xsd#CommitmentsAndContingenciesTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/CondensedConsolidatedBalanceSheets" xlink:href="ftnt-20190930.xsd#CondensedConsolidatedBalanceSheets" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/CondensedConsolidatedBalanceSheetsParenthetical" xlink:href="ftnt-20190930.xsd#CondensedConsolidatedBalanceSheetsParenthetical" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/CondensedConsolidatedStatementsOfCashFlows" xlink:href="ftnt-20190930.xsd#CondensedConsolidatedStatementsOfCashFlows" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/CondensedConsolidatedStatementsOfComprehensiveIncome" xlink:href="ftnt-20190930.xsd#CondensedConsolidatedStatementsOfComprehensiveIncome" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/CondensedConsolidatedStatementsOfIncome" xlink:href="ftnt-20190930.xsd#CondensedConsolidatedStatementsOfIncome" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/CondensedConsolidatedStatementsOfStockholdersEquity" xlink:href="ftnt-20190930.xsd#CondensedConsolidatedStatementsOfStockholdersEquity" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/DefinedContributionPlans" xlink:href="ftnt-20190930.xsd#DefinedContributionPlans" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/DefinedContributionPlansDetails" xlink:href="ftnt-20190930.xsd#DefinedContributionPlansDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/DocumentAndEntityInformation" xlink:href="ftnt-20190930.xsd#DocumentAndEntityInformation" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/FinancialInstrumentsAndFairValue" xlink:href="ftnt-20190930.xsd#FinancialInstrumentsAndFairValue" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/FinancialInstrumentsAndFairValueFairValueMeasurementsDetails" xlink:href="ftnt-20190930.xsd#FinancialInstrumentsAndFairValueFairValueMeasurementsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/FinancialInstrumentsAndFairValueInvestmentsDetails" xlink:href="ftnt-20190930.xsd#FinancialInstrumentsAndFairValueInvestmentsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/FinancialInstrumentsAndFairValueTables" xlink:href="ftnt-20190930.xsd#FinancialInstrumentsAndFairValueTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/GoodwillAndOtherIntangibleAssetsNet" xlink:href="ftnt-20190930.xsd#GoodwillAndOtherIntangibleAssetsNet" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/GoodwillAndOtherIntangibleAssetsNetEstimatedFutureAmortizationExpenseDetails" xlink:href="ftnt-20190930.xsd#GoodwillAndOtherIntangibleAssetsNetEstimatedFutureAmortizationExpenseDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/GoodwillAndOtherIntangibleAssetsNetNarrativeDetails" xlink:href="ftnt-20190930.xsd#GoodwillAndOtherIntangibleAssetsNetNarrativeDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/GoodwillAndOtherIntangibleAssetsNetOtherIntangibleAssetsNetDetails" xlink:href="ftnt-20190930.xsd#GoodwillAndOtherIntangibleAssetsNetOtherIntangibleAssetsNetDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/GoodwillAndOtherIntangibleAssetsNetTables" xlink:href="ftnt-20190930.xsd#GoodwillAndOtherIntangibleAssetsNetTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/IncomeTaxes" xlink:href="ftnt-20190930.xsd#IncomeTaxes" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/IncomeTaxesNarrativeDetails" xlink:href="ftnt-20190930.xsd#IncomeTaxesNarrativeDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/Inventory" xlink:href="ftnt-20190930.xsd#Inventory" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/InventoryDetails" xlink:href="ftnt-20190930.xsd#InventoryDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/InventoryTables" xlink:href="ftnt-20190930.xsd#InventoryTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/InvestmentsInPrivatelyHeldCompanies" xlink:href="ftnt-20190930.xsd#InvestmentsInPrivatelyHeldCompanies" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/InvestmentsInPrivatelyHeldCompaniesDetails" xlink:href="ftnt-20190930.xsd#InvestmentsInPrivatelyHeldCompaniesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/Leases" xlink:href="ftnt-20190930.xsd#Leases" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/LeasesComponentsOfLeaseExpenseDetails" xlink:href="ftnt-20190930.xsd#LeasesComponentsOfLeaseExpenseDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/LeasesDetails" xlink:href="ftnt-20190930.xsd#LeasesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/LeasesFuturePrincipalContractualObligationsForOperatingLeaseCommitmentsDetails" xlink:href="ftnt-20190930.xsd#LeasesFuturePrincipalContractualObligationsForOperatingLeaseCommitmentsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/LeasesMaturitiesOfOperatingLeaseLiabilitiesDetails" xlink:href="ftnt-20190930.xsd#LeasesMaturitiesOfOperatingLeaseLiabilitiesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/LeasesSupplementalBalanceSheetInformationDetails" xlink:href="ftnt-20190930.xsd#LeasesSupplementalBalanceSheetInformationDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/LeasesSupplementalCashFlowInformationDetails" xlink:href="ftnt-20190930.xsd#LeasesSupplementalCashFlowInformationDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/LeasesTables" xlink:href="ftnt-20190930.xsd#LeasesTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/NetIncomePerShare" xlink:href="ftnt-20190930.xsd#NetIncomePerShare" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/NetIncomePerShareAntiDilutiveSecuritiesDetails" xlink:href="ftnt-20190930.xsd#NetIncomePerShareAntiDilutiveSecuritiesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/NetIncomePerShareCalculationOfBasicAndDilutedDetails" xlink:href="ftnt-20190930.xsd#NetIncomePerShareCalculationOfBasicAndDilutedDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/NetIncomePerShareTables" xlink:href="ftnt-20190930.xsd#NetIncomePerShareTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/PropertyAndEquipmentnet" xlink:href="ftnt-20190930.xsd#PropertyAndEquipmentnet" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/PropertyAndEquipmentnetDetails" xlink:href="ftnt-20190930.xsd#PropertyAndEquipmentnetDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/PropertyAndEquipmentnetTables" xlink:href="ftnt-20190930.xsd#PropertyAndEquipmentnetTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/RevenueRecognition" xlink:href="ftnt-20190930.xsd#RevenueRecognition" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/RevenueRecognitionDetails" xlink:href="ftnt-20190930.xsd#RevenueRecognitionDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/RevenueRecognitionDisaggregatedRevenuesDetails" xlink:href="ftnt-20190930.xsd#RevenueRecognitionDisaggregatedRevenuesDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/RevenueRecognitionPerformanceObligationSatisfactionPeriodDetails" xlink:href="ftnt-20190930.xsd#RevenueRecognitionPerformanceObligationSatisfactionPeriodDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/RevenueRecognitionTables" xlink:href="ftnt-20190930.xsd#RevenueRecognitionTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/SegmentInformation" xlink:href="ftnt-20190930.xsd#SegmentInformation" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/SegmentInformationDetails" xlink:href="ftnt-20190930.xsd#SegmentInformationDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/SegmentInformationTables" xlink:href="ftnt-20190930.xsd#SegmentInformationTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/StockholdersEquity" xlink:href="ftnt-20190930.xsd#StockholdersEquity" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/StockholdersEquityAllocationOfStockBasedCompensationExpenseDetails" xlink:href="ftnt-20190930.xsd#StockholdersEquityAllocationOfStockBasedCompensationExpenseDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/StockholdersEquityDetails" xlink:href="ftnt-20190930.xsd#StockholdersEquityDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/StockholdersEquityRestrictedStockUnitsActivityDetails" xlink:href="ftnt-20190930.xsd#StockholdersEquityRestrictedStockUnitsActivityDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/StockholdersEquityShareRepurchaseProgramDetails" xlink:href="ftnt-20190930.xsd#StockholdersEquityShareRepurchaseProgramDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/StockholdersEquityTables" xlink:href="ftnt-20190930.xsd#StockholdersEquityTables" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/SubsequentEvents" xlink:href="ftnt-20190930.xsd#SubsequentEvents" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/SubsequentEventsDetails" xlink:href="ftnt-20190930.xsd#SubsequentEventsDetails" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/SummaryOfSignificantAccountingPolicies" xlink:href="ftnt-20190930.xsd#SummaryOfSignificantAccountingPolicies" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/SummaryOfSignificantAccountingPoliciesPolicies" xlink:href="ftnt-20190930.xsd#SummaryOfSignificantAccountingPoliciesPolicies" xlink:type="simple" />
  <link:roleRef roleURI="http://www.fortinet.com/role/SummaryOfSignificantAccountingPoliciesRecentAccountingPronouncementsDetails" xlink:href="ftnt-20190930.xsd#SummaryOfSignificantAccountingPoliciesRecentAccountingPronouncementsDetails" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-default" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#dimension-domain" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/domain-member" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#domain-member" xlink:type="simple" />
  <link:arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#hypercube-dimension" xlink:type="simple" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/AccumulatedOtherComprehensiveIncomeLoss" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/AccumulatedOtherComprehensiveIncomeLossChangesInAccumulatedBalancesOfOtherComprehensiveLossDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_C7F3E3549F77A5B705FF73A622FD5F73" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_2A4C36CFE11B74605F4773A622FCB03F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_C7F3E3549F77A5B705FF73A622FD5F73" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_2A4C36CFE11B74605F4773A622FCB03F" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_6B17A49497E2E30F65B173A622FC663F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_2A4C36CFE11B74605F4773A622FCB03F" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_6B17A49497E2E30F65B173A622FC663F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_85FE65450A606758DFEC73A622FD2513_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_6B17A49497E2E30F65B173A622FC663F" xlink:to="loc_us-gaap_EquityComponentDomain_85FE65450A606758DFEC73A622FD2513_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_85FE65450A606758DFEC73A622FD2513" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_6B17A49497E2E30F65B173A622FC663F" xlink:to="loc_us-gaap_EquityComponentDomain_85FE65450A606758DFEC73A622FD2513" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember" xlink:label="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_61E07311BF2FB29922B873A622FD3616" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_85FE65450A606758DFEC73A622FD2513" xlink:to="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_61E07311BF2FB29922B873A622FD3616" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_AccumulatedOtherComprehensiveIncomeRollForward" xlink:label="loc_ftnt_AccumulatedOtherComprehensiveIncomeRollForward_52531F0EDFAC23CCCD9373A622FE3FD4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_C7F3E3549F77A5B705FF73A622FD5F73" xlink:to="loc_ftnt_AccumulatedOtherComprehensiveIncomeRollForward_52531F0EDFAC23CCCD9373A622FE3FD4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1_F63C513E3CD59709F8D273A622FE2524" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_AccumulatedOtherComprehensiveIncomeRollForward_52531F0EDFAC23CCCD9373A622FE3FD4" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1_F63C513E3CD59709F8D273A622FE2524" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax_1F58F210A2D8FEDD9BB673A622FE0DC2" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_AccumulatedOtherComprehensiveIncomeRollForward_52531F0EDFAC23CCCD9373A622FE3FD4" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax_1F58F210A2D8FEDD9BB673A622FE0DC2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax" xlink:label="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax_261C3530BC4628365BE073A622FED5A8" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_AccumulatedOtherComprehensiveIncomeRollForward_52531F0EDFAC23CCCD9373A622FE3FD4" xlink:to="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax_261C3530BC4628365BE073A622FED5A8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeTax_A2452A2F22E20E892B6273A622FF2D5A" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_AccumulatedOtherComprehensiveIncomeRollForward_52531F0EDFAC23CCCD9373A622FE3FD4" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeTax_A2452A2F22E20E892B6273A622FF2D5A" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_AccumulatedOtherComprehensiveIncomeTaxRollForward" xlink:label="loc_ftnt_AccumulatedOtherComprehensiveIncomeTaxRollForward_CE3A7E87B29AE3A582A873A622FFD800" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_C7F3E3549F77A5B705FF73A622FD5F73" xlink:to="loc_ftnt_AccumulatedOtherComprehensiveIncomeTaxRollForward_CE3A7E87B29AE3A582A873A622FFD800" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_AccumulatedOtherComprehensiveIncomeLossTax" xlink:label="loc_ftnt_AccumulatedOtherComprehensiveIncomeLossTax_34820013A2599CAD1F3C73A6230014D1" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_AccumulatedOtherComprehensiveIncomeTaxRollForward_CE3A7E87B29AE3A582A873A622FFD800" xlink:to="loc_ftnt_AccumulatedOtherComprehensiveIncomeLossTax_34820013A2599CAD1F3C73A6230014D1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsTax_642F3CEB7D769F4EDC0573A623005FB4" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_AccumulatedOtherComprehensiveIncomeTaxRollForward_CE3A7E87B29AE3A582A873A622FFD800" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsTax_642F3CEB7D769F4EDC0573A623005FB4" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_ReclassificationfromAccumulatedOtherComprehensiveIncomeCurrentPeriodTax" xlink:label="loc_ftnt_ReclassificationfromAccumulatedOtherComprehensiveIncomeCurrentPeriodTax_D1D99455941387AACAEE73A623009A8F" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_AccumulatedOtherComprehensiveIncomeTaxRollForward_CE3A7E87B29AE3A582A873A622FFD800" xlink:to="loc_ftnt_ReclassificationfromAccumulatedOtherComprehensiveIncomeCurrentPeriodTax_D1D99455941387AACAEE73A623009A8F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossTax_7590B2B78E48FB13499D73A62300ABB0" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_AccumulatedOtherComprehensiveIncomeTaxRollForward_CE3A7E87B29AE3A582A873A622FFD800" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossTax_7590B2B78E48FB13499D73A62300ABB0" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/AccumulatedOtherComprehensiveIncomeLossTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/BusinessCombinations" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/BusinessCombinationsAdditionalInformationDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_51CD2D3F37C9FE95F8DE73A622830F1E" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_7F57D54B35B2F6DAEB3273A62281C2F9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_51CD2D3F37C9FE95F8DE73A622830F1E" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_7F57D54B35B2F6DAEB3273A62281C2F9" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_63060758B13DE6E0C60273A6228232E2" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_7F57D54B35B2F6DAEB3273A62281C2F9" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_63060758B13DE6E0C60273A6228232E2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_2E37805809C248D4954573A62282BFD5_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_63060758B13DE6E0C60273A6228232E2" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_2E37805809C248D4954573A62282BFD5_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_2E37805809C248D4954573A62282BFD5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_63060758B13DE6E0C60273A6228232E2" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_2E37805809C248D4954573A62282BFD5" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_ZoneFoxHoldingsLimitedMember" xlink:label="loc_ftnt_ZoneFoxHoldingsLimitedMember_E92B6918CACA2D1593C973A62283AD0D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_2E37805809C248D4954573A62282BFD5" xlink:to="loc_ftnt_ZoneFoxHoldingsLimitedMember_E92B6918CACA2D1593C973A62283AD0D" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_BradfordNetworksInc.Member" xlink:label="loc_ftnt_BradfordNetworksInc.Member_E6B411C3C5F6BD245A8173A622830BA3" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_2E37805809C248D4954573A62282BFD5" xlink:to="loc_ftnt_BradfordNetworksInc.Member_E6B411C3C5F6BD245A8173A622830BA3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_DA322163251DC25E306773A622832315" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_51CD2D3F37C9FE95F8DE73A622830F1E" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredGoodwillAndLiabilitiesAssumedNet_DA322163251DC25E306773A622832315" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_Goodwill" xlink:label="loc_us-gaap_Goodwill_9E4D82AFEDDA2538713D73A622846A71" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_51CD2D3F37C9FE95F8DE73A622830F1E" xlink:to="loc_us-gaap_Goodwill_9E4D82AFEDDA2538713D73A622846A71" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_4639310919EE3E1274AC73A6228405A8" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_51CD2D3F37C9FE95F8DE73A622830F1E" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibleAssetsOtherThanGoodwill_4639310919EE3E1274AC73A6228405A8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_B67E169D6CCEFF3F6CED73A6228454DC" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_51CD2D3F37C9FE95F8DE73A622830F1E" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_B67E169D6CCEFF3F6CED73A6228454DC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_F7146AD962EF42E52FDB73A6228413B5" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_51CD2D3F37C9FE95F8DE73A622830F1E" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_F7146AD962EF42E52FDB73A6228413B5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationLiability" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_187272084A157DBDCC4473A62284C2B2" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_51CD2D3F37C9FE95F8DE73A622830F1E" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationLiability_187272084A157DBDCC4473A62284C2B2" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/CommitmentsAndContingencies" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/CommitmentsAndContingenciesDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/CommitmentsAndContingenciesMinimumOperatingLeasePaymentsDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/CommitmentsAndContingenciesTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/CondensedConsolidatedBalanceSheets" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_C028F25B01198EA2717C0370B195E1CA" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_A824666C83479ACEBC670370B195E106" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_C028F25B01198EA2717C0370B195E1CA" xlink:to="loc_us-gaap_StatementTable_A824666C83479ACEBC670370B195E106" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_StatementScenarioAxis" xlink:label="loc_srt_StatementScenarioAxis_52277A7BD12E5A8374160370B195003F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_A824666C83479ACEBC670370B195E106" xlink:to="loc_srt_StatementScenarioAxis_52277A7BD12E5A8374160370B195003F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="loc_srt_ScenarioUnspecifiedDomain_01B42535F75AC8E2AAC20370B1958F8A_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementScenarioAxis_52277A7BD12E5A8374160370B195003F" xlink:to="loc_srt_ScenarioUnspecifiedDomain_01B42535F75AC8E2AAC20370B1958F8A_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="loc_srt_ScenarioUnspecifiedDomain_01B42535F75AC8E2AAC20370B1958F8A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementScenarioAxis_52277A7BD12E5A8374160370B195003F" xlink:to="loc_srt_ScenarioUnspecifiedDomain_01B42535F75AC8E2AAC20370B1958F8A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AssetsAbstract" xlink:label="loc_us-gaap_AssetsAbstract_032247785B5BBCA6320F0370B196C803" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_C028F25B01198EA2717C0370B195E1CA" xlink:to="loc_us-gaap_AssetsAbstract_032247785B5BBCA6320F0370B196C803" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AssetsCurrentAbstract" xlink:label="loc_us-gaap_AssetsCurrentAbstract_8CB5E308EDB005B104C00370B19645C4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_032247785B5BBCA6320F0370B196C803" xlink:to="loc_us-gaap_AssetsCurrentAbstract_8CB5E308EDB005B104C00370B19645C4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_4FCB64A7D2B7278D49080370B196057A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_8CB5E308EDB005B104C00370B19645C4" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_4FCB64A7D2B7278D49080370B196057A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShortTermInvestments" xlink:label="loc_us-gaap_ShortTermInvestments_316C0831BAF2FD347FDF0370B197A46E" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_8CB5E308EDB005B104C00370B19645C4" xlink:to="loc_us-gaap_ShortTermInvestments_316C0831BAF2FD347FDF0370B197A46E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccountsReceivableNetCurrent" xlink:label="loc_us-gaap_AccountsReceivableNetCurrent_ABD78C5E2FC8F67663A80370B197A225" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_8CB5E308EDB005B104C00370B19645C4" xlink:to="loc_us-gaap_AccountsReceivableNetCurrent_ABD78C5E2FC8F67663A80370B197A225" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_InventoryNet" xlink:label="loc_us-gaap_InventoryNet_EC33B72F47D3629EBEDB0370B1974EB3" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_8CB5E308EDB005B104C00370B19645C4" xlink:to="loc_us-gaap_InventoryNet_EC33B72F47D3629EBEDB0370B1974EB3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PrepaidExpenseAndOtherAssetsCurrent" xlink:label="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_731061856BAB3FB1B1F30370B19758F1" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_8CB5E308EDB005B104C00370B19645C4" xlink:to="loc_us-gaap_PrepaidExpenseAndOtherAssetsCurrent_731061856BAB3FB1B1F30370B19758F1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AssetsCurrent" xlink:label="loc_us-gaap_AssetsCurrent_CAF684124B8C532D49110370B19709DE" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsCurrentAbstract_8CB5E308EDB005B104C00370B19645C4" xlink:to="loc_us-gaap_AssetsCurrent_CAF684124B8C532D49110370B19709DE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_LongTermInvestments" xlink:label="loc_us-gaap_LongTermInvestments_5C95E177E84FBD89A0740370B1979A24" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_032247785B5BBCA6320F0370B196C803" xlink:to="loc_us-gaap_LongTermInvestments_5C95E177E84FBD89A0740370B1979A24" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_A5CF6CD2351C5B3480290370B198E2D5" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_032247785B5BBCA6320F0370B196C803" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_A5CF6CD2351C5B3480290370B198E2D5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CapitalizedContractCostNet" xlink:label="loc_us-gaap_CapitalizedContractCostNet_98A48B3768A394D92E140370B198F3B4" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_032247785B5BBCA6320F0370B196C803" xlink:to="loc_us-gaap_CapitalizedContractCostNet_98A48B3768A394D92E140370B198F3B4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_DeferredIncomeTaxAssetsNet" xlink:label="loc_us-gaap_DeferredIncomeTaxAssetsNet_0EF08D46A35E2F5BF20C0370B198BCA8" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_032247785B5BBCA6320F0370B196C803" xlink:to="loc_us-gaap_DeferredIncomeTaxAssetsNet_0EF08D46A35E2F5BF20C0370B198BCA8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_Goodwill" xlink:label="loc_us-gaap_Goodwill_BC2F52FEBABAB1DE2B520370B1989D44" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_032247785B5BBCA6320F0370B196C803" xlink:to="loc_us-gaap_Goodwill_BC2F52FEBABAB1DE2B520370B1989D44" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill" xlink:label="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_2A719AD7B663A7B184660370B1980BF2" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_032247785B5BBCA6320F0370B196C803" xlink:to="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_2A719AD7B663A7B184660370B1980BF2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OtherAssetsNoncurrent" xlink:label="loc_us-gaap_OtherAssetsNoncurrent_B6A3AE6C060B876E59580370B19813A4" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_032247785B5BBCA6320F0370B196C803" xlink:to="loc_us-gaap_OtherAssetsNoncurrent_B6A3AE6C060B876E59580370B19813A4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_Assets" xlink:label="loc_us-gaap_Assets_C660E7FA84E2CF6278EB0370B1991D71" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AssetsAbstract_032247785B5BBCA6320F0370B196C803" xlink:to="loc_us-gaap_Assets_C660E7FA84E2CF6278EB0370B1991D71" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_LiabilitiesAndStockholdersEquityAbstract" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_6A84A1E25FF5815E26530370B1995902" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_C028F25B01198EA2717C0370B195E1CA" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_6A84A1E25FF5815E26530370B1995902" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_LiabilitiesCurrentAbstract" xlink:label="loc_us-gaap_LiabilitiesCurrentAbstract_D901EC52AEFF4C205AAD0370B1990934" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_6A84A1E25FF5815E26530370B1995902" xlink:to="loc_us-gaap_LiabilitiesCurrentAbstract_D901EC52AEFF4C205AAD0370B1990934" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccountsPayableCurrent" xlink:label="loc_us-gaap_AccountsPayableCurrent_DE0565966526E5EC03830370B1997117" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_D901EC52AEFF4C205AAD0370B1990934" xlink:to="loc_us-gaap_AccountsPayableCurrent_DE0565966526E5EC03830370B1997117" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccruedLiabilitiesCurrent" xlink:label="loc_us-gaap_AccruedLiabilitiesCurrent_CAEAEBB75AB09AF920BC0370B199C81C" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_D901EC52AEFF4C205AAD0370B1990934" xlink:to="loc_us-gaap_AccruedLiabilitiesCurrent_CAEAEBB75AB09AF920BC0370B199C81C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EmployeeRelatedLiabilitiesCurrent" xlink:label="loc_us-gaap_EmployeeRelatedLiabilitiesCurrent_49C92E8C5DB78AF371AE0370B199586B" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_D901EC52AEFF4C205AAD0370B1990934" xlink:to="loc_us-gaap_EmployeeRelatedLiabilitiesCurrent_49C92E8C5DB78AF371AE0370B199586B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccruedIncomeTaxesCurrent" xlink:label="loc_us-gaap_AccruedIncomeTaxesCurrent_5B2724911D9A0D0B24110370B19A4105" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_D901EC52AEFF4C205AAD0370B1990934" xlink:to="loc_us-gaap_AccruedIncomeTaxesCurrent_5B2724911D9A0D0B24110370B19A4105" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ContractWithCustomerLiabilityCurrent" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityCurrent_AC09944B1447C8DD5D900370B19AB7DB" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_D901EC52AEFF4C205AAD0370B1990934" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityCurrent_AC09944B1447C8DD5D900370B19AB7DB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_LiabilitiesCurrent" xlink:label="loc_us-gaap_LiabilitiesCurrent_0036B590E0FAA72D8F9A0370B19A0965" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesCurrentAbstract_D901EC52AEFF4C205AAD0370B1990934" xlink:to="loc_us-gaap_LiabilitiesCurrent_0036B590E0FAA72D8F9A0370B19A0965" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ContractWithCustomerLiabilityNoncurrent" xlink:label="loc_us-gaap_ContractWithCustomerLiabilityNoncurrent_51C3A3CD87309B9A7FF00370B19AB6E9" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_6A84A1E25FF5815E26530370B1995902" xlink:to="loc_us-gaap_ContractWithCustomerLiabilityNoncurrent_51C3A3CD87309B9A7FF00370B19AB6E9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccruedIncomeTaxesNoncurrent" xlink:label="loc_us-gaap_AccruedIncomeTaxesNoncurrent_89EDA4E03F44F570EC8F0370B19A892C" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_6A84A1E25FF5815E26530370B1995902" xlink:to="loc_us-gaap_AccruedIncomeTaxesNoncurrent_89EDA4E03F44F570EC8F0370B19A892C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OtherLiabilitiesNoncurrent" xlink:label="loc_us-gaap_OtherLiabilitiesNoncurrent_E67AC36DC4A3BB4699C70370B19A80AC" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_6A84A1E25FF5815E26530370B1995902" xlink:to="loc_us-gaap_OtherLiabilitiesNoncurrent_E67AC36DC4A3BB4699C70370B19A80AC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_Liabilities" xlink:label="loc_us-gaap_Liabilities_67A355EE55D6BFE0731B0370B19B1E49" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_6A84A1E25FF5815E26530370B1995902" xlink:to="loc_us-gaap_Liabilities_67A355EE55D6BFE0731B0370B19B1E49" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CommitmentsAndContingencies" xlink:label="loc_us-gaap_CommitmentsAndContingencies_C7D2B11B3247048BEA440370B19BDE64" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_6A84A1E25FF5815E26530370B1995902" xlink:to="loc_us-gaap_CommitmentsAndContingencies_C7D2B11B3247048BEA440370B19BDE64" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StockholdersEquityAbstract" xlink:label="loc_us-gaap_StockholdersEquityAbstract_94D8592629867BC814830370B19B8E63" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_6A84A1E25FF5815E26530370B1995902" xlink:to="loc_us-gaap_StockholdersEquityAbstract_94D8592629867BC814830370B19B8E63" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CommonStockValue" xlink:label="loc_us-gaap_CommonStockValue_2CC97BA74E66D96E90390370B19B78CC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_94D8592629867BC814830370B19B8E63" xlink:to="loc_us-gaap_CommonStockValue_2CC97BA74E66D96E90390370B19B78CC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AdditionalPaidInCapitalCommonStock" xlink:label="loc_us-gaap_AdditionalPaidInCapitalCommonStock_24BB9536C4C9AE80A8B90370B19BF5F6" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_94D8592629867BC814830370B19B8E63" xlink:to="loc_us-gaap_AdditionalPaidInCapitalCommonStock_24BB9536C4C9AE80A8B90370B19BF5F6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_B70C89A66B6D25F098FC0370B19B5898" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_94D8592629867BC814830370B19B8E63" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossNetOfTax_B70C89A66B6D25F098FC0370B19B5898" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RetainedEarningsAccumulatedDeficit" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_479069FBFDE3E87EB7930370B19CD45F" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_94D8592629867BC814830370B19B8E63" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_479069FBFDE3E87EB7930370B19CD45F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StockholdersEquity" xlink:label="loc_us-gaap_StockholdersEquity_802538886106ED5EB0270370B19CBF79" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StockholdersEquityAbstract_94D8592629867BC814830370B19B8E63" xlink:to="loc_us-gaap_StockholdersEquity_802538886106ED5EB0270370B19CBF79" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_LiabilitiesAndStockholdersEquity" xlink:label="loc_us-gaap_LiabilitiesAndStockholdersEquity_4F4B9C1A181A8246BBA20370B19C34DD" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LiabilitiesAndStockholdersEquityAbstract_6A84A1E25FF5815E26530370B1995902" xlink:to="loc_us-gaap_LiabilitiesAndStockholdersEquity_4F4B9C1A181A8246BBA20370B19C34DD" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/CondensedConsolidatedBalanceSheetsParenthetical" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_FA7D9C14B14752A60F6773A62310DDBE" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_719CE71566B2B4DBECA573A6230B79E1" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_FA7D9C14B14752A60F6773A62310DDBE" xlink:to="loc_us-gaap_StatementTable_719CE71566B2B4DBECA573A6230B79E1" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_StatementScenarioAxis" xlink:label="loc_srt_StatementScenarioAxis_83EB7808CFAF0E343D7773A6230B417A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_719CE71566B2B4DBECA573A6230B79E1" xlink:to="loc_srt_StatementScenarioAxis_83EB7808CFAF0E343D7773A6230B417A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="loc_srt_ScenarioUnspecifiedDomain_FC60CD87C53984C417FC73A6230B2A5B_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementScenarioAxis_83EB7808CFAF0E343D7773A6230B417A" xlink:to="loc_srt_ScenarioUnspecifiedDomain_FC60CD87C53984C417FC73A6230B2A5B_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="loc_srt_ScenarioUnspecifiedDomain_FC60CD87C53984C417FC73A6230B2A5B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementScenarioAxis_83EB7808CFAF0E343D7773A6230B417A" xlink:to="loc_srt_ScenarioUnspecifiedDomain_FC60CD87C53984C417FC73A6230B2A5B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CommonStockParOrStatedValuePerShare" xlink:label="loc_us-gaap_CommonStockParOrStatedValuePerShare_B71A18B2A7BD710B6DB373A62311F977" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_FA7D9C14B14752A60F6773A62310DDBE" xlink:to="loc_us-gaap_CommonStockParOrStatedValuePerShare_B71A18B2A7BD710B6DB373A62311F977" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CommonStockSharesAuthorized" xlink:label="loc_us-gaap_CommonStockSharesAuthorized_365C8EC159F34E69DE0973A623118FB0" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_FA7D9C14B14752A60F6773A62310DDBE" xlink:to="loc_us-gaap_CommonStockSharesAuthorized_365C8EC159F34E69DE0973A623118FB0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CommonStockSharesIssued" xlink:label="loc_us-gaap_CommonStockSharesIssued_DA467964268B2EEB854673A623113EF0" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_FA7D9C14B14752A60F6773A62310DDBE" xlink:to="loc_us-gaap_CommonStockSharesIssued_DA467964268B2EEB854673A623113EF0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CommonStockSharesOutstanding" xlink:label="loc_us-gaap_CommonStockSharesOutstanding_855E9A95855AA3ACC6B073A623112F0D" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_FA7D9C14B14752A60F6773A62310DDBE" xlink:to="loc_us-gaap_CommonStockSharesOutstanding_855E9A95855AA3ACC6B073A623112F0D" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/CondensedConsolidatedStatementsOfCashFlows" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_522E1A3DF1A7995B30F20370B1606A75" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_26DB1F2394BDCB3E6CBE0370B15F40D5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_522E1A3DF1A7995B30F20370B1606A75" xlink:to="loc_us-gaap_StatementTable_26DB1F2394BDCB3E6CBE0370B15F40D5" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_StatementScenarioAxis" xlink:label="loc_srt_StatementScenarioAxis_A1DEA060E0CCC43174490370B15F51F3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_26DB1F2394BDCB3E6CBE0370B15F40D5" xlink:to="loc_srt_StatementScenarioAxis_A1DEA060E0CCC43174490370B15F51F3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="loc_srt_ScenarioUnspecifiedDomain_8E2D920D2CE92C3AAF5D0370B15F5BA9_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementScenarioAxis_A1DEA060E0CCC43174490370B15F51F3" xlink:to="loc_srt_ScenarioUnspecifiedDomain_8E2D920D2CE92C3AAF5D0370B15F5BA9_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="loc_srt_ScenarioUnspecifiedDomain_8E2D920D2CE92C3AAF5D0370B15F5BA9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementScenarioAxis_A1DEA060E0CCC43174490370B15F51F3" xlink:to="loc_srt_ScenarioUnspecifiedDomain_8E2D920D2CE92C3AAF5D0370B15F5BA9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_CD18A12C685F98CDC7170370B1609FDA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_522E1A3DF1A7995B30F20370B1606A75" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_CD18A12C685F98CDC7170370B1609FDA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_D39B2EFED2DF50CD6ABE0370B16007CB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_CD18A12C685F98CDC7170370B1609FDA" xlink:to="loc_us-gaap_NetIncomeLoss_D39B2EFED2DF50CD6ABE0370B16007CB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract" xlink:label="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_170928619ED901BF2D770370B1600381" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_CD18A12C685F98CDC7170370B1609FDA" xlink:to="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_170928619ED901BF2D770370B1600381" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensation" xlink:label="loc_us-gaap_ShareBasedCompensation_26572A105D8CC7339B420370B16123FA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_170928619ED901BF2D770370B1600381" xlink:to="loc_us-gaap_ShareBasedCompensation_26572A105D8CC7339B420370B16123FA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CapitalizedContractCostAmortization" xlink:label="loc_us-gaap_CapitalizedContractCostAmortization_40764AA45B842B43D9C60370B161DD77" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_170928619ED901BF2D770370B1600381" xlink:to="loc_us-gaap_CapitalizedContractCostAmortization_40764AA45B842B43D9C60370B161DD77" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_DepreciationDepletionAndAmortization" xlink:label="loc_us-gaap_DepreciationDepletionAndAmortization_733893F186E0A663A5EF0370B161D840" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_170928619ED901BF2D770370B1600381" xlink:to="loc_us-gaap_DepreciationDepletionAndAmortization_733893F186E0A663A5EF0370B161D840" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments" xlink:label="loc_us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments_228468B3C87D91D01F1A0370B16138D4" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_170928619ED901BF2D770370B1600381" xlink:to="loc_us-gaap_AccretionAmortizationOfDiscountsAndPremiumsInvestments_228468B3C87D91D01F1A0370B16138D4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OtherNoncashIncomeExpense" xlink:label="loc_us-gaap_OtherNoncashIncomeExpense_99651D915183036EFB900370B1624EBE" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AdjustmentsToReconcileNetIncomeLossToCashProvidedByUsedInOperatingActivitiesAbstract_170928619ED901BF2D770370B1600381" xlink:to="loc_us-gaap_OtherNoncashIncomeExpense_99651D915183036EFB900370B1624EBE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract" xlink:label="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract_58D0DBA323351A983B950370B162491A" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_CD18A12C685F98CDC7170370B1609FDA" xlink:to="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract_58D0DBA323351A983B950370B162491A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncreaseDecreaseInAccountsReceivable" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_BF7A929194AF7D10C9FA0370B1627B91" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract_58D0DBA323351A983B950370B162491A" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsReceivable_BF7A929194AF7D10C9FA0370B1627B91" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncreaseDecreaseInInventories" xlink:label="loc_us-gaap_IncreaseDecreaseInInventories_8AA4E7A46DE9557881570370B1622639" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract_58D0DBA323351A983B950370B162491A" xlink:to="loc_us-gaap_IncreaseDecreaseInInventories_8AA4E7A46DE9557881570370B1622639" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets" xlink:label="loc_us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_DFFA31B31D56D859078A0370B162900E" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract_58D0DBA323351A983B950370B162491A" xlink:to="loc_us-gaap_IncreaseDecreaseInPrepaidDeferredExpenseAndOtherAssets_DFFA31B31D56D859078A0370B162900E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncreaseDecreaseInDeferredCharges" xlink:label="loc_us-gaap_IncreaseDecreaseInDeferredCharges_856FCF44EEECDB2404520370B1620072" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract_58D0DBA323351A983B950370B162491A" xlink:to="loc_us-gaap_IncreaseDecreaseInDeferredCharges_856FCF44EEECDB2404520370B1620072" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncreaseDecreaseInDeferredIncomeTaxes" xlink:label="loc_us-gaap_IncreaseDecreaseInDeferredIncomeTaxes_8112EDB3439AB5A9827E0370B163C508" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract_58D0DBA323351A983B950370B162491A" xlink:to="loc_us-gaap_IncreaseDecreaseInDeferredIncomeTaxes_8112EDB3439AB5A9827E0370B163C508" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncreaseDecreaseInOtherOperatingAssets" xlink:label="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssets_28A602BF665EA7DACC270370B163AA98" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract_58D0DBA323351A983B950370B162491A" xlink:to="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssets_28A602BF665EA7DACC270370B163AA98" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncreaseDecreaseInAccountsPayable" xlink:label="loc_us-gaap_IncreaseDecreaseInAccountsPayable_7F4877A91AB5276F869E0370B163CDCB" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract_58D0DBA323351A983B950370B162491A" xlink:to="loc_us-gaap_IncreaseDecreaseInAccountsPayable_7F4877A91AB5276F869E0370B163CDCB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncreaseDecreaseInAccruedLiabilities" xlink:label="loc_us-gaap_IncreaseDecreaseInAccruedLiabilities_9C5BC6AC8478C2FE9A330370B1634673" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract_58D0DBA323351A983B950370B162491A" xlink:to="loc_us-gaap_IncreaseDecreaseInAccruedLiabilities_9C5BC6AC8478C2FE9A330370B1634673" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncreaseDecreaseInEmployeeRelatedLiabilities" xlink:label="loc_us-gaap_IncreaseDecreaseInEmployeeRelatedLiabilities_632A4A95879F88F42EE00370B16327B3" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract_58D0DBA323351A983B950370B162491A" xlink:to="loc_us-gaap_IncreaseDecreaseInEmployeeRelatedLiabilities_632A4A95879F88F42EE00370B16327B3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncreaseDecreaseInAccruedLiabilitiesAndOtherOperatingLiabilities" xlink:label="loc_us-gaap_IncreaseDecreaseInAccruedLiabilitiesAndOtherOperatingLiabilities_A311A5FD5ED3E7FB1B8D0370B163DA5F" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract_58D0DBA323351A983B950370B162491A" xlink:to="loc_us-gaap_IncreaseDecreaseInAccruedLiabilitiesAndOtherOperatingLiabilities_A311A5FD5ED3E7FB1B8D0370B163DA5F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncreaseDecreaseInContractWithCustomerLiability" xlink:label="loc_us-gaap_IncreaseDecreaseInContractWithCustomerLiability_20DA9EE7CA386B71E7740370B164D14B" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract_58D0DBA323351A983B950370B162491A" xlink:to="loc_us-gaap_IncreaseDecreaseInContractWithCustomerLiability_20DA9EE7CA386B71E7740370B164D14B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncreaseDecreaseInAccruedIncomeTaxesPayable" xlink:label="loc_us-gaap_IncreaseDecreaseInAccruedIncomeTaxesPayable_F2361D52BB3C6D07A76C0370B1640863" xlink:type="locator" />
    <link:definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInOtherOperatingAssetsAndLiabilitiesNetAbstract_58D0DBA323351A983B950370B162491A" xlink:to="loc_us-gaap_IncreaseDecreaseInAccruedIncomeTaxesPayable_F2361D52BB3C6D07A76C0370B1640863" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NetCashProvidedByUsedInOperatingActivities" xlink:label="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_F87B23C18C32674E228D0370B16484E1" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInOperatingActivitiesAbstract_CD18A12C685F98CDC7170370B1609FDA" xlink:to="loc_us-gaap_NetCashProvidedByUsedInOperatingActivities_F87B23C18C32674E228D0370B16484E1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_38D1C1A0CBDD6EA384A00370B164114C" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_522E1A3DF1A7995B30F20370B1606A75" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_38D1C1A0CBDD6EA384A00370B164114C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PaymentsToAcquireInvestments" xlink:label="loc_us-gaap_PaymentsToAcquireInvestments_8A035198C1708F58FEB70370B164FC58" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_38D1C1A0CBDD6EA384A00370B164114C" xlink:to="loc_us-gaap_PaymentsToAcquireInvestments_8A035198C1708F58FEB70370B164FC58" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt" xlink:label="loc_us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt_5A7F9EA2F931CB22E6940370B16467D0" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_38D1C1A0CBDD6EA384A00370B164114C" xlink:to="loc_us-gaap_ProceedsFromSaleOfAvailableForSaleSecuritiesDebt_5A7F9EA2F931CB22E6940370B16467D0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities" xlink:label="loc_us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities_D801ADABA720EA83791D0370B165696D" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_38D1C1A0CBDD6EA384A00370B164114C" xlink:to="loc_us-gaap_ProceedsFromMaturitiesPrepaymentsAndCallsOfAvailableForSaleSecurities_D801ADABA720EA83791D0370B165696D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PaymentsToAcquirePropertyPlantAndEquipment" xlink:label="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_237C2FCE9581D892C6870370B165FDEC" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_38D1C1A0CBDD6EA384A00370B164114C" xlink:to="loc_us-gaap_PaymentsToAcquirePropertyPlantAndEquipment_237C2FCE9581D892C6870370B165FDEC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_365A488D3292641E26CC0370B1659A19" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_38D1C1A0CBDD6EA384A00370B164114C" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_365A488D3292641E26CC0370B1659A19" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NetCashProvidedByUsedInInvestingActivities" xlink:label="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_8116FA25C235812139A00370B1656CE8" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInInvestingActivitiesAbstract_38D1C1A0CBDD6EA384A00370B164114C" xlink:to="loc_us-gaap_NetCashProvidedByUsedInInvestingActivities_8116FA25C235812139A00370B1656CE8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_E1609D727EB5363984630370B1653FFB" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_522E1A3DF1A7995B30F20370B1606A75" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_E1609D727EB5363984630370B1653FFB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PaymentsForRepurchaseOfCommonStock" xlink:label="loc_us-gaap_PaymentsForRepurchaseOfCommonStock_888DEF057C507403BDBC0370B165B623" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_E1609D727EB5363984630370B1653FFB" xlink:to="loc_us-gaap_PaymentsForRepurchaseOfCommonStock_888DEF057C507403BDBC0370B165B623" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ProceedsFromIssuanceOfCommonStock" xlink:label="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_40A31C9773B47D85D68E0370B16617B1" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_E1609D727EB5363984630370B1653FFB" xlink:to="loc_us-gaap_ProceedsFromIssuanceOfCommonStock_40A31C9773B47D85D68E0370B16617B1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation" xlink:label="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_A56C8AD72426992A39D10370B166E619" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_E1609D727EB5363984630370B1653FFB" xlink:to="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_A56C8AD72426992A39D10370B166E619" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RepaymentsOfAssumedDebt" xlink:label="loc_us-gaap_RepaymentsOfAssumedDebt_152952A3009C02131B4D0370B16677EF" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_E1609D727EB5363984630370B1653FFB" xlink:to="loc_us-gaap_RepaymentsOfAssumedDebt_152952A3009C02131B4D0370B16677EF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NetCashProvidedByUsedInFinancingActivities" xlink:label="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_ACEDC55199707985FE690370B166F5F0" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetCashProvidedByUsedInFinancingActivitiesAbstract_E1609D727EB5363984630370B1653FFB" xlink:to="loc_us-gaap_NetCashProvidedByUsedInFinancingActivities_ACEDC55199707985FE690370B166F5F0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease" xlink:label="loc_us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease_DE7B3BFB9DD0504D26F00370B1660DE4" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_522E1A3DF1A7995B30F20370B1606A75" xlink:to="loc_us-gaap_CashAndCashEquivalentsPeriodIncreaseDecrease_DE7B3BFB9DD0504D26F00370B1660DE4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CashAndCashEquivalentsAtCarryingValue" xlink:label="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_E83530DCA4ECE56A7FF00370B1672238" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_522E1A3DF1A7995B30F20370B1606A75" xlink:to="loc_us-gaap_CashAndCashEquivalentsAtCarryingValue_E83530DCA4ECE56A7FF00370B1672238" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SupplementalCashFlowInformationAbstract" xlink:label="loc_us-gaap_SupplementalCashFlowInformationAbstract_A75829660C508EEA14350370B167FA99" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_522E1A3DF1A7995B30F20370B1606A75" xlink:to="loc_us-gaap_SupplementalCashFlowInformationAbstract_A75829660C508EEA14350370B167FA99" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_LeaseLiabilityArisingfromObtainingRightofUseAsset" xlink:label="loc_ftnt_LeaseLiabilityArisingfromObtainingRightofUseAsset_E0FC75A2E6D9C2F526100370B167EAC5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_A75829660C508EEA14350370B167FA99" xlink:to="loc_ftnt_LeaseLiabilityArisingfromObtainingRightofUseAsset_E0FC75A2E6D9C2F526100370B167EAC5" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_RepurchaseofCommonStockSettlementofLiabilitiesIncurred" xlink:label="loc_ftnt_RepurchaseofCommonStockSettlementofLiabilitiesIncurred_BDA84AD6DFAD193BD2B10370B167E0EE" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SupplementalCashFlowInformationAbstract_A75829660C508EEA14350370B167FA99" xlink:to="loc_ftnt_RepurchaseofCommonStockSettlementofLiabilitiesIncurred_BDA84AD6DFAD193BD2B10370B167E0EE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract" xlink:label="loc_us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract_4BC25FCEB51A6995A7F90370B16841AC" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_522E1A3DF1A7995B30F20370B1606A75" xlink:to="loc_us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract_4BC25FCEB51A6995A7F90370B16841AC" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_TransfersofEvaluationUnitsfromInventorytoPropertyandEquipment" xlink:label="loc_ftnt_TransfersofEvaluationUnitsfromInventorytoPropertyandEquipment_CF162D1C709415470B5B0370B1681240" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract_4BC25FCEB51A6995A7F90370B16841AC" xlink:to="loc_ftnt_TransfersofEvaluationUnitsfromInventorytoPropertyandEquipment_CF162D1C709415470B5B0370B1681240" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CapitalExpendituresIncurredButNotYetPaid" xlink:label="loc_us-gaap_CapitalExpendituresIncurredButNotYetPaid_7F4ECF0C7B56393CB61F0370B1684482" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract_4BC25FCEB51A6995A7F90370B16841AC" xlink:to="loc_us-gaap_CapitalExpendituresIncurredButNotYetPaid_7F4ECF0C7B56393CB61F0370B1684482" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_RepurchaseofCommonStockLiabilitiesIncurred" xlink:label="loc_ftnt_RepurchaseofCommonStockLiabilitiesIncurred_37D7144A89C8143AA6000370B1686D67" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CashFlowNoncashInvestingAndFinancingActivitiesDisclosureAbstract_4BC25FCEB51A6995A7F90370B16841AC" xlink:to="loc_ftnt_RepurchaseofCommonStockLiabilitiesIncurred_37D7144A89C8143AA6000370B1686D67" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/CondensedConsolidatedStatementsOfComprehensiveIncome" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_B9FA4023827F110B9AB203CE29C188CA" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_761EB042F3156F9BED7203CE29C05D60" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_B9FA4023827F110B9AB203CE29C188CA" xlink:to="loc_us-gaap_StatementTable_761EB042F3156F9BED7203CE29C05D60" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_StatementScenarioAxis" xlink:label="loc_srt_StatementScenarioAxis_80019C405D25C65359BC03CE29C097E2" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_761EB042F3156F9BED7203CE29C05D60" xlink:to="loc_srt_StatementScenarioAxis_80019C405D25C65359BC03CE29C097E2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="loc_srt_ScenarioUnspecifiedDomain_FCC41D761DE04AB786EC03CE29C12B55_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementScenarioAxis_80019C405D25C65359BC03CE29C097E2" xlink:to="loc_srt_ScenarioUnspecifiedDomain_FCC41D761DE04AB786EC03CE29C12B55_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_ScenarioUnspecifiedDomain" xlink:label="loc_srt_ScenarioUnspecifiedDomain_FCC41D761DE04AB786EC03CE29C12B55" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementScenarioAxis_80019C405D25C65359BC03CE29C097E2" xlink:to="loc_srt_ScenarioUnspecifiedDomain_FCC41D761DE04AB786EC03CE29C12B55" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_D5FB90B4209D1CE01EF203CE29C1A56E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_B9FA4023827F110B9AB203CE29C188CA" xlink:to="loc_us-gaap_NetIncomeLoss_D5FB90B4209D1CE01EF203CE29C1A56E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeTaxPortionAttributableToParentAbstract" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeTaxPortionAttributableToParentAbstract_B516C6744A43EA61CE2103CE29C1D2DB" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_B9FA4023827F110B9AB203CE29C188CA" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeTaxPortionAttributableToParentAbstract_B516C6744A43EA61CE2103CE29C1D2DB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax_4DED08A3D830D9CA40F503CE29C1417D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossBeforeTaxPortionAttributableToParentAbstract_B516C6744A43EA61CE2103CE29C1D2DB" xlink:to="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodBeforeTax_4DED08A3D830D9CA40F503CE29C1417D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1_7C9E96C426CADD32733A03CE29C10149" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossBeforeTaxPortionAttributableToParentAbstract_B516C6744A43EA61CE2103CE29C1D2DB" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossTaxPortionAttributableToParent1_7C9E96C426CADD32733A03CE29C10149" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_54059ABD18DCAFBEDBAB03CE29C163A7" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherComprehensiveIncomeLossBeforeTaxPortionAttributableToParentAbstract_B516C6744A43EA61CE2103CE29C1D2DB" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossNetOfTaxPortionAttributableToParent_54059ABD18DCAFBEDBAB03CE29C163A7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ComprehensiveIncomeNetOfTax" xlink:label="loc_us-gaap_ComprehensiveIncomeNetOfTax_89A4DBC54E3B86E5221C03CE29C1C0E2" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_B9FA4023827F110B9AB203CE29C188CA" xlink:to="loc_us-gaap_ComprehensiveIncomeNetOfTax_89A4DBC54E3B86E5221C03CE29C1C0E2" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/CondensedConsolidatedStatementsOfIncome" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_C35D50BC81EBA976549C0370B1854A31" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_A51996D7EADEE4D34ED00370B18410DC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_C35D50BC81EBA976549C0370B1854A31" xlink:to="loc_us-gaap_StatementTable_A51996D7EADEE4D34ED00370B18410DC" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_ProductOrServiceAxis" xlink:label="loc_srt_ProductOrServiceAxis_341E0E36A78E34F6A18F0370B184B62A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_A51996D7EADEE4D34ED00370B18410DC" xlink:to="loc_srt_ProductOrServiceAxis_341E0E36A78E34F6A18F0370B184B62A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_8079DF04D080C1B275090370B184B709_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_341E0E36A78E34F6A18F0370B184B62A" xlink:to="loc_srt_ProductsAndServicesDomain_8079DF04D080C1B275090370B184B709_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_8079DF04D080C1B275090370B184B709" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_341E0E36A78E34F6A18F0370B184B62A" xlink:to="loc_srt_ProductsAndServicesDomain_8079DF04D080C1B275090370B184B709" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ProductMember" xlink:label="loc_us-gaap_ProductMember_A5D03993AC44D0D7E1480370B184117B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_8079DF04D080C1B275090370B184B709" xlink:to="loc_us-gaap_ProductMember_A5D03993AC44D0D7E1480370B184117B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ServiceMember" xlink:label="loc_us-gaap_ServiceMember_6796F3BCD84FFADA44C60370B185C75B" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_8079DF04D080C1B275090370B184B709" xlink:to="loc_us-gaap_ServiceMember_6796F3BCD84FFADA44C60370B185C75B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RevenueFromContractWithCustomerAbstract" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerAbstract_B694D65CDFEC22571BAA0370B185FDDE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_C35D50BC81EBA976549C0370B1854A31" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerAbstract_B694D65CDFEC22571BAA0370B185FDDE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_B293BC68BA730F4093A60370B1851E11" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueFromContractWithCustomerAbstract_B694D65CDFEC22571BAA0370B185FDDE" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_B293BC68BA730F4093A60370B1851E11" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CostOfGoodsAndServicesSoldAbstract" xlink:label="loc_us-gaap_CostOfGoodsAndServicesSoldAbstract_D47763DC82C6855B87890370B186C681" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_C35D50BC81EBA976549C0370B1854A31" xlink:to="loc_us-gaap_CostOfGoodsAndServicesSoldAbstract_D47763DC82C6855B87890370B186C681" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CostOfGoodsAndServicesSold" xlink:label="loc_us-gaap_CostOfGoodsAndServicesSold_8DDE4DA285F3B04320E80370B186F65C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_CostOfGoodsAndServicesSoldAbstract_D47763DC82C6855B87890370B186C681" xlink:to="loc_us-gaap_CostOfGoodsAndServicesSold_8DDE4DA285F3B04320E80370B186F65C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_GrossProfitAbstract" xlink:label="loc_us-gaap_GrossProfitAbstract_B97A63667306A17494450370B18655B2" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_C35D50BC81EBA976549C0370B1854A31" xlink:to="loc_us-gaap_GrossProfitAbstract_B97A63667306A17494450370B18655B2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_GrossProfit" xlink:label="loc_us-gaap_GrossProfit_A73C4112F4A3A01B1AB20370B1860E4F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_GrossProfitAbstract_B97A63667306A17494450370B18655B2" xlink:to="loc_us-gaap_GrossProfit_A73C4112F4A3A01B1AB20370B1860E4F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OperatingExpensesAbstract" xlink:label="loc_us-gaap_OperatingExpensesAbstract_C80E34CABF05589278180370B1862876" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_C35D50BC81EBA976549C0370B1854A31" xlink:to="loc_us-gaap_OperatingExpensesAbstract_C80E34CABF05589278180370B1862876" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ResearchAndDevelopmentExpense" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpense_019AEE2E45F1F10970AE0370B1875534" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_C80E34CABF05589278180370B1862876" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpense_019AEE2E45F1F10970AE0370B1875534" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SellingAndMarketingExpense" xlink:label="loc_us-gaap_SellingAndMarketingExpense_401461E953F933158E4F0370B1873848" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_C80E34CABF05589278180370B1862876" xlink:to="loc_us-gaap_SellingAndMarketingExpense_401461E953F933158E4F0370B1873848" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_GeneralAndAdministrativeExpense" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpense_85D55FE8B0438A3F247A0370B187EE95" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_C80E34CABF05589278180370B1862876" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpense_85D55FE8B0438A3F247A0370B187EE95" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OperatingExpenses" xlink:label="loc_us-gaap_OperatingExpenses_E8FF4621F22B478A53430370B18750F6" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OperatingExpensesAbstract_C80E34CABF05589278180370B1862876" xlink:to="loc_us-gaap_OperatingExpenses_E8FF4621F22B478A53430370B18750F6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OperatingIncomeLoss" xlink:label="loc_us-gaap_OperatingIncomeLoss_7C40A20D0E96F1CFC5C70370B1870F1D" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_C35D50BC81EBA976549C0370B1854A31" xlink:to="loc_us-gaap_OperatingIncomeLoss_7C40A20D0E96F1CFC5C70370B1870F1D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_InvestmentIncomeNet" xlink:label="loc_us-gaap_InvestmentIncomeNet_E3CE3B71418878E0EA440370B188F8EA" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_C35D50BC81EBA976549C0370B1854A31" xlink:to="loc_us-gaap_InvestmentIncomeNet_E3CE3B71418878E0EA440370B188F8EA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OtherNonoperatingIncomeExpense" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_A4DB4D200847E1583DDD0370B1883B17" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_C35D50BC81EBA976549C0370B1854A31" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_A4DB4D200847E1583DDD0370B1883B17" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments" xlink:label="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments_DDD13AF823A51AA65AD10370B1886EF0" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_C35D50BC81EBA976549C0370B1854A31" xlink:to="loc_us-gaap_IncomeLossFromContinuingOperationsBeforeIncomeTaxesMinorityInterestAndIncomeLossFromEquityMethodInvestments_DDD13AF823A51AA65AD10370B1886EF0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_57BE1496343AEB27B3B30370B188070E" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_C35D50BC81EBA976549C0370B1854A31" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_57BE1496343AEB27B3B30370B188070E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_5217D409EB89A580DB9B0370B18825E2" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_C35D50BC81EBA976549C0370B1854A31" xlink:to="loc_us-gaap_NetIncomeLoss_5217D409EB89A580DB9B0370B18825E2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EarningsPerShareAbstract" xlink:label="loc_us-gaap_EarningsPerShareAbstract_4E62F789AC108939C3110370B188AE56" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_C35D50BC81EBA976549C0370B1854A31" xlink:to="loc_us-gaap_EarningsPerShareAbstract_4E62F789AC108939C3110370B188AE56" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EarningsPerShareBasic" xlink:label="loc_us-gaap_EarningsPerShareBasic_A6BD0591EF8915BDFF120370B189C146" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareAbstract_4E62F789AC108939C3110370B188AE56" xlink:to="loc_us-gaap_EarningsPerShareBasic_A6BD0591EF8915BDFF120370B189C146" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EarningsPerShareDiluted" xlink:label="loc_us-gaap_EarningsPerShareDiluted_E5D04D347E69D267E5250370B189FD93" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EarningsPerShareAbstract_4E62F789AC108939C3110370B188AE56" xlink:to="loc_us-gaap_EarningsPerShareDiluted_E5D04D347E69D267E5250370B189FD93" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_E8690AF648D1126134C30370B189B4CD" xlink:type="locator" />
    <link:definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_C35D50BC81EBA976549C0370B1854A31" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_E8690AF648D1126134C30370B189B4CD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_E47D99A6B07F90F696720370B189D299" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_E8690AF648D1126134C30370B189B4CD" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_E47D99A6B07F90F696720370B189D299" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_ACEEA33FD92BCD38265C0370B189E8B8" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_E8690AF648D1126134C30370B189B4CD" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_ACEEA33FD92BCD38265C0370B189E8B8" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/CondensedConsolidatedStatementsOfStockholdersEquity" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StatementLineItems" xlink:label="loc_us-gaap_StatementLineItems_D118A2C674AF2B8C643F0370B172D84F" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StatementTable" xlink:label="loc_us-gaap_StatementTable_1057A6E169AA2DE7CE160370B171C856" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_StatementLineItems_D118A2C674AF2B8C643F0370B172D84F" xlink:to="loc_us-gaap_StatementTable_1057A6E169AA2DE7CE160370B171C856" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_773AD4E7351C1F75F6370370B171448A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_StatementTable_1057A6E169AA2DE7CE160370B171C856" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_773AD4E7351C1F75F6370370B171448A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_FA5EC8E939EF11AAAF8C0370B1713E43_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_773AD4E7351C1F75F6370370B171448A" xlink:to="loc_us-gaap_EquityComponentDomain_FA5EC8E939EF11AAAF8C0370B1713E43_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_FA5EC8E939EF11AAAF8C0370B1713E43" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_773AD4E7351C1F75F6370370B171448A" xlink:to="loc_us-gaap_EquityComponentDomain_FA5EC8E939EF11AAAF8C0370B1713E43" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CommonStockMember" xlink:label="loc_us-gaap_CommonStockMember_4D02930C4A812B40AB070370B1723A83" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_FA5EC8E939EF11AAAF8C0370B1713E43" xlink:to="loc_us-gaap_CommonStockMember_4D02930C4A812B40AB070370B1723A83" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AdditionalPaidInCapitalMember" xlink:label="loc_us-gaap_AdditionalPaidInCapitalMember_FFF0C1EE73F32F76100C0370B1725679" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_FA5EC8E939EF11AAAF8C0370B1713E43" xlink:to="loc_us-gaap_AdditionalPaidInCapitalMember_FFF0C1EE73F32F76100C0370B1725679" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_E23BA62045A6F4CF25C20370B1728620" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_FA5EC8E939EF11AAAF8C0370B1713E43" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_E23BA62045A6F4CF25C20370B1728620" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RetainedEarningsMember" xlink:label="loc_us-gaap_RetainedEarningsMember_9F24F7A9D4C494C41C560370B172E012" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_FA5EC8E939EF11AAAF8C0370B1713E43" xlink:to="loc_us-gaap_RetainedEarningsMember_9F24F7A9D4C494C41C560370B172E012" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncreaseDecreaseInStockholdersEquityRollForward" xlink:label="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_A550B6E3E42C19ED180D0370B1734DB3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_StatementLineItems_D118A2C674AF2B8C643F0370B172D84F" xlink:to="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_A550B6E3E42C19ED180D0370B1734DB3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SharesIssued" xlink:label="loc_us-gaap_SharesIssued_9B3F98C43382EB848D6F0370B17391D8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_A550B6E3E42C19ED180D0370B1734DB3" xlink:to="loc_us-gaap_SharesIssued_9B3F98C43382EB848D6F0370B17391D8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StockholdersEquity" xlink:label="loc_us-gaap_StockholdersEquity_2360F4266965CA2D3E070370B1733E33" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_A550B6E3E42C19ED180D0370B1734DB3" xlink:to="loc_us-gaap_StockholdersEquity_2360F4266965CA2D3E070370B1733E33" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_StockIssuedDuringPeriodSharesEquityIncentivePlans" xlink:label="loc_ftnt_StockIssuedDuringPeriodSharesEquityIncentivePlans_DFF0B183F312195D0C9E0370B17363E0" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_A550B6E3E42C19ED180D0370B1734DB3" xlink:to="loc_ftnt_StockIssuedDuringPeriodSharesEquityIncentivePlans_DFF0B183F312195D0C9E0370B17363E0" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_StockIssuedDuringPeriodValueEquityIncentivePlansNetofTaxesWithheld" xlink:label="loc_ftnt_StockIssuedDuringPeriodValueEquityIncentivePlansNetofTaxesWithheld_2844D144D2CC1FD96EDA0370B173854C" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_A550B6E3E42C19ED180D0370B1734DB3" xlink:to="loc_ftnt_StockIssuedDuringPeriodValueEquityIncentivePlansNetofTaxesWithheld_2844D144D2CC1FD96EDA0370B173854C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_0CD9390C1B50A14BA3AC0370B174FF7F" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_A550B6E3E42C19ED180D0370B1734DB3" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_0CD9390C1B50A14BA3AC0370B174FF7F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_CE7F5A54A0D84B012AFB0370B1746DBC" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_A550B6E3E42C19ED180D0370B1734DB3" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_CE7F5A54A0D84B012AFB0370B1746DBC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue" xlink:label="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_E68E355203A89B63E92B0370B174A8A5" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_A550B6E3E42C19ED180D0370B1734DB3" xlink:to="loc_us-gaap_AdjustmentsToAdditionalPaidInCapitalSharebasedCompensationRequisiteServicePeriodRecognitionValue_E68E355203A89B63E92B0370B174A8A5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption" xlink:label="loc_us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption_DF9EEBE9698FB332AD680370B174A08E" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_A550B6E3E42C19ED180D0370B1734DB3" xlink:to="loc_us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption_DF9EEBE9698FB332AD680370B174A08E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_97068CE9DDC5038AFB900370B174DCF3" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_A550B6E3E42C19ED180D0370B1734DB3" xlink:to="loc_us-gaap_OtherComprehensiveIncomeUnrealizedHoldingGainLossOnSecuritiesArisingDuringPeriodNetOfTax_97068CE9DDC5038AFB900370B174DCF3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_A86599C6C6D7E18B4ABE0370B174FD63" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncreaseDecreaseInStockholdersEquityRollForward_A550B6E3E42C19ED180D0370B1734DB3" xlink:to="loc_us-gaap_NetIncomeLoss_A86599C6C6D7E18B4ABE0370B174FD63" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/DefinedContributionPlans" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/DefinedContributionPlansDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/DocumentAndEntityInformation" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/FinancialInstrumentsAndFairValue" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/FinancialInstrumentsAndFairValueFairValueMeasurementsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_635854AF1E6FC212CCF973A62572D9FD" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_A83C7E06E22460E5E66A73A6256B2342" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_635854AF1E6FC212CCF973A62572D9FD" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_A83C7E06E22460E5E66A73A6256B2342" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueByMeasurementFrequencyAxis" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_007753110E6DED5D2E2B73A6256BFBD9" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_A83C7E06E22460E5E66A73A6256B2342" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_007753110E6DED5D2E2B73A6256BFBD9" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_2E7ECB344D1EC440BFF473A6256B6102_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_007753110E6DED5D2E2B73A6256BFBD9" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_2E7ECB344D1EC440BFF473A6256B6102_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_2E7ECB344D1EC440BFF473A6256B6102" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_007753110E6DED5D2E2B73A6256BFBD9" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_2E7ECB344D1EC440BFF473A6256B6102" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueMeasurementsRecurringMember" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_478DD4BCF33695CB11C573A6256CD37F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_2E7ECB344D1EC440BFF473A6256B6102" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_478DD4BCF33695CB11C573A6256CD37F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueByMeasurementBasisAxis" xlink:label="loc_us-gaap_FairValueByMeasurementBasisAxis_32C1A0140ADB2B6CCA5773A6256CEE11" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_A83C7E06E22460E5E66A73A6256B2342" xlink:to="loc_us-gaap_FairValueByMeasurementBasisAxis_32C1A0140ADB2B6CCA5773A6256CEE11" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_212E1836BC5E6E56B9F573A6256D806D_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_32C1A0140ADB2B6CCA5773A6256CEE11" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_212E1836BC5E6E56B9F573A6256D806D_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueDisclosureItemAmountsDomain" xlink:label="loc_us-gaap_FairValueDisclosureItemAmountsDomain_38BB2EACB02566F7284073A6256CC6D3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_32C1A0140ADB2B6CCA5773A6256CEE11" xlink:to="loc_us-gaap_FairValueDisclosureItemAmountsDomain_38BB2EACB02566F7284073A6256CC6D3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_212E1836BC5E6E56B9F573A6256D806D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_38BB2EACB02566F7284073A6256CC6D3" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_212E1836BC5E6E56B9F573A6256D806D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember" xlink:label="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_88F7A94901B7A4C3167B73A6256D5703" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_38BB2EACB02566F7284073A6256CC6D3" xlink:to="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_88F7A94901B7A4C3167B73A6256D5703" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CarryingReportedAmountFairValueDisclosureMember" xlink:label="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_E296A8791C2F30065A6B73A6256D5F50" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_38BB2EACB02566F7284073A6256CC6D3" xlink:to="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_E296A8791C2F30065A6B73A6256D5F50" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="loc_us-gaap_FinancialInstrumentAxis_C3FB23E8B6F09414E13673A6256DEBA8" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_A83C7E06E22460E5E66A73A6256B2342" xlink:to="loc_us-gaap_FinancialInstrumentAxis_C3FB23E8B6F09414E13673A6256DEBA8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_234966E50C837BA7E1FF73A6256E7599_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_C3FB23E8B6F09414E13673A6256DEBA8" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_234966E50C837BA7E1FF73A6256E7599_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_234966E50C837BA7E1FF73A6256E7599" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_C3FB23E8B6F09414E13673A6256DEBA8" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_234966E50C837BA7E1FF73A6256E7599" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CorporateDebtSecuritiesMember" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_EA7974470AE95820BF5073A6256EDA73" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_234966E50C837BA7E1FF73A6256E7599" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_EA7974470AE95820BF5073A6256EDA73" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember" xlink:label="loc_us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_B93A3674B4614B15528673A6256FD7D5" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_234966E50C837BA7E1FF73A6256E7599" xlink:to="loc_us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_B93A3674B4614B15528673A6256FD7D5" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_CertificatesofDepositandTermDepositsMember" xlink:label="loc_ftnt_CertificatesofDepositandTermDepositsMember_AB69F340C3F4D434A3C673A6256F8133" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_234966E50C837BA7E1FF73A6256E7599" xlink:to="loc_ftnt_CertificatesofDepositandTermDepositsMember_AB69F340C3F4D434A3C673A6256F8133" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_USTreasuryAndGovernmentMember" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_21ABB602FF0F19FF9CB773A6256FF742" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_234966E50C837BA7E1FF73A6256E7599" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_21ABB602FF0F19FF9CB773A6256FF742" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_MoneyMarketFundsExcludingCommercialPaperMember" xlink:label="loc_ftnt_MoneyMarketFundsExcludingCommercialPaperMember_05DE562360CCB3BA972C73A6256FB9E6" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_234966E50C837BA7E1FF73A6256E7599" xlink:to="loc_ftnt_MoneyMarketFundsExcludingCommercialPaperMember_05DE562360CCB3BA972C73A6256FB9E6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_9DF15F93646D132357DE73A62570544C" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_A83C7E06E22460E5E66A73A6256B2342" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_9DF15F93646D132357DE73A62570544C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_4B4EDD69DBB09005D87F73A62570867E_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_9DF15F93646D132357DE73A62570544C" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_4B4EDD69DBB09005D87F73A62570867E_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_4B4EDD69DBB09005D87F73A62570867E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_9DF15F93646D132357DE73A62570544C" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_4B4EDD69DBB09005D87F73A62570867E" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_CashEquivalentsLineItemMember" xlink:label="loc_ftnt_CashEquivalentsLineItemMember_10F39537C1D75F20570F73A62570BF1C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_4B4EDD69DBB09005D87F73A62570867E" xlink:to="loc_ftnt_CashEquivalentsLineItemMember_10F39537C1D75F20570F73A62570BF1C" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_ShortTermInvestmentsLineItemMember" xlink:label="loc_ftnt_ShortTermInvestmentsLineItemMember_28BC44531FBBCDE9862073A62570767D" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_4B4EDD69DBB09005D87F73A62570867E" xlink:to="loc_ftnt_ShortTermInvestmentsLineItemMember_28BC44531FBBCDE9862073A62570767D" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_LongTermInvestmentsLineItemMember" xlink:label="loc_ftnt_LongTermInvestmentsLineItemMember_ACE8167DA5E04543878A73A6257107F2" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_4B4EDD69DBB09005D87F73A62570867E" xlink:to="loc_ftnt_LongTermInvestmentsLineItemMember_ACE8167DA5E04543878A73A6257107F2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_1D8A15B8D4B38591113673A62571D888" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_A83C7E06E22460E5E66A73A6256B2342" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_1D8A15B8D4B38591113673A62571D888" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_A72A445D0CE4A3D78DAA73A6257120F8_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_1D8A15B8D4B38591113673A62571D888" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_A72A445D0CE4A3D78DAA73A6257120F8_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_A72A445D0CE4A3D78DAA73A6257120F8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_1D8A15B8D4B38591113673A62571D888" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_A72A445D0CE4A3D78DAA73A6257120F8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueInputsLevel1Member" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_AF292509C70A3C51156473A62571CBF7" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_A72A445D0CE4A3D78DAA73A6257120F8" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_AF292509C70A3C51156473A62571CBF7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_6DCFCB24309D91A81F7E73A625728A3F" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_A72A445D0CE4A3D78DAA73A6257120F8" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_6DCFCB24309D91A81F7E73A625728A3F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FairValueInputsLevel3Member" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_0B71AD7D6B435A218E8573A625728657" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_A72A445D0CE4A3D78DAA73A6257120F8" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_0B71AD7D6B435A218E8573A625728657" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_2931F0413854FA1A8DCF73A62572DDF0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_635854AF1E6FC212CCF973A62572D9FD" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_2931F0413854FA1A8DCF73A62572DDF0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AssetsFairValueDisclosure" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_4066AF7A02E29C21EE5A73A6257362C8" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_635854AF1E6FC212CCF973A62572D9FD" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_4066AF7A02E29C21EE5A73A6257362C8" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/FinancialInstrumentsAndFairValueInvestmentsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_863CF716E7D0BF6A9A5B0370B1063250" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesTable" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesTable_43123EFFE43A2DB4B3430370B104ED63" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_863CF716E7D0BF6A9A5B0370B1063250" xlink:to="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesTable_43123EFFE43A2DB4B3430370B104ED63" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FinancialInstrumentAxis" xlink:label="loc_us-gaap_FinancialInstrumentAxis_801685BB2B9ED7024BD70370B1041751" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesTable_43123EFFE43A2DB4B3430370B104ED63" xlink:to="loc_us-gaap_FinancialInstrumentAxis_801685BB2B9ED7024BD70370B1041751" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_EC05AC0BE394500F417E0370B10587E5_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FinancialInstrumentAxis_801685BB2B9ED7024BD70370B1041751" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_EC05AC0BE394500F417E0370B10587E5_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain" xlink:label="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_EC05AC0BE394500F417E0370B10587E5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FinancialInstrumentAxis_801685BB2B9ED7024BD70370B1041751" xlink:to="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_EC05AC0BE394500F417E0370B10587E5" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CorporateDebtSecuritiesMember" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_8BA238E2D70F6FF143F50370B105F029" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_EC05AC0BE394500F417E0370B10587E5" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_8BA238E2D70F6FF143F50370B105F029" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember" xlink:label="loc_us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_8A0E7721E3470BA614230370B10564C1" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_EC05AC0BE394500F417E0370B10587E5" xlink:to="loc_us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_8A0E7721E3470BA614230370B10564C1" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_CertificatesofDepositandTermDepositsMember" xlink:label="loc_ftnt_CertificatesofDepositandTermDepositsMember_D0519C9167771474F31A0370B105F87F" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_EC05AC0BE394500F417E0370B10587E5" xlink:to="loc_ftnt_CertificatesofDepositandTermDepositsMember_D0519C9167771474F31A0370B105F87F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_USTreasuryAndGovernmentMember" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_5AD788475CFEC9042C280370B106AA98" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TransfersAndServicingOfFinancialInstrumentsTypesOfFinancialInstrumentsDomain_EC05AC0BE394500F417E0370B10587E5" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_5AD788475CFEC9042C280370B106AA98" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_E9EAA3123B73A3253F370370B106EB6C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_863CF716E7D0BF6A9A5B0370B1063250" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_E9EAA3123B73A3253F370370B106EB6C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_5439AF905EBBAA5192850370B1061B8C" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_863CF716E7D0BF6A9A5B0370B1063250" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_5439AF905EBBAA5192850370B1061B8C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_44C83F74DCFEA7197E4B0370B1061B49" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_863CF716E7D0BF6A9A5B0370B1063250" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_44C83F74DCFEA7197E4B0370B1061B49" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_51A3B6F87E46BEB24EF90370B10682F6" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_863CF716E7D0BF6A9A5B0370B1063250" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_51A3B6F87E46BEB24EF90370B10682F6" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract" xlink:label="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_F7C9322AE37C3C713D190370B1063376" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_863CF716E7D0BF6A9A5B0370B1063250" xlink:to="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_F7C9322AE37C3C713D190370B1063376" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue_75F284FAAA34F43BE00A0370B1074F30" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_F7C9322AE37C3C713D190370B1063376" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue_75F284FAAA34F43BE00A0370B1074F30" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_FF44F842222B7A6DFD3E0370B10793FD" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_F7C9322AE37C3C713D190370B1063376" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_FF44F842222B7A6DFD3E0370B10793FD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue_022D834AD694ABE0ABD10370B1073720" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_F7C9322AE37C3C713D190370B1063376" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue_022D834AD694ABE0ABD10370B1073720" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_851A5CB47E053DB82BAB0370B107C724" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_F7C9322AE37C3C713D190370B1063376" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_851A5CB47E053DB82BAB0370B107C724" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue_ECDBA591285CE5979E370370B107C209" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_F7C9322AE37C3C713D190370B1063376" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue_ECDBA591285CE5979E370370B107C209" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss_DEB77BCC06EDDEAFC7680370B1081CD1" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_DebtSecuritiesAvailableforsaleContinuousUnrealizedLossPositionAbstract_F7C9322AE37C3C713D190370B1063376" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss_DEB77BCC06EDDEAFC7680370B1081CD1" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_49FEC8F8EAE6A8CE86700370B10892AE" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_863CF716E7D0BF6A9A5B0370B1063250" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_49FEC8F8EAE6A8CE86700370B10892AE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue_68F18E75B90FF66DBEA80370B108D73D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_49FEC8F8EAE6A8CE86700370B10892AE" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue_68F18E75B90FF66DBEA80370B108D73D" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_AvailableForSaleSecuritiesDebtMaturitiesAfterOneYearFairValue" xlink:label="loc_ftnt_AvailableForSaleSecuritiesDebtMaturitiesAfterOneYearFairValue_F615FF2B38AAFDC018230370B108105D" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_49FEC8F8EAE6A8CE86700370B10892AE" xlink:to="loc_ftnt_AvailableForSaleSecuritiesDebtMaturitiesAfterOneYearFairValue_F615FF2B38AAFDC018230370B108105D" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/FinancialInstrumentsAndFairValueTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/GoodwillAndOtherIntangibleAssetsNet" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/GoodwillAndOtherIntangibleAssetsNetEstimatedFutureAmortizationExpenseDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/GoodwillAndOtherIntangibleAssetsNetNarrativeDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/GoodwillAndOtherIntangibleAssetsNetOtherIntangibleAssetsNetDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_00E90E647CEB137174E10370B0B9492D" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_784C7996916D87F76C220370B0B80BB2" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_00E90E647CEB137174E10370B0B9492D" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_784C7996916D87F76C220370B0B80BB2" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_58F94498ACD03BC380330370B0B8A2BD" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_784C7996916D87F76C220370B0B80BB2" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_58F94498ACD03BC380330370B0B8A2BD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_641CD514784504E9F9A30370B0B898E7_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_58F94498ACD03BC380330370B0B8A2BD" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_641CD514784504E9F9A30370B0B898E7_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_641CD514784504E9F9A30370B0B898E7" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_58F94498ACD03BC380330370B0B8A2BD" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_641CD514784504E9F9A30370B0B898E7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_DevelopedTechnologyRightsMember" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_1CDF1ABF4D76C97CAD720370B0B98BA6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_641CD514784504E9F9A30370B0B898E7" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_1CDF1ABF4D76C97CAD720370B0B98BA6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CustomerRelationshipsMember" xlink:label="loc_us-gaap_CustomerRelationshipsMember_B2E5698A8ED6CC66884E0370B0B96D61" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_641CD514784504E9F9A30370B0B898E7" xlink:to="loc_us-gaap_CustomerRelationshipsMember_B2E5698A8ED6CC66884E0370B0B96D61" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_0B40269464F6B9A4E5730370B0B915ED" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_00E90E647CEB137174E10370B0B9492D" xlink:to="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_0B40269464F6B9A4E5730370B0B915ED" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsNetAbstract" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_C646CFC2D1A84A5D8B4C0370B0BA5C3E" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_00E90E647CEB137174E10370B0B9492D" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_C646CFC2D1A84A5D8B4C0370B0BA5C3E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsGross" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_D3AA96F43070D639D3330370B0BA2D1C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_C646CFC2D1A84A5D8B4C0370B0BA5C3E" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_D3AA96F43070D639D3330370B0BA2D1C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_70CA8DAD37FFA1E0028D0370B0BAEDD8" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_C646CFC2D1A84A5D8B4C0370B0BA5C3E" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_70CA8DAD37FFA1E0028D0370B0BAEDD8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsNet" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_D8D1C5D78157134330D10370B0BA9B21" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_C646CFC2D1A84A5D8B4C0370B0BA5C3E" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_D8D1C5D78157134330D10370B0BA9B21" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/GoodwillAndOtherIntangibleAssetsNetTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/IncomeTaxes" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/IncomeTaxesNarrativeDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/Inventory" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/InventoryDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/InventoryTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/InvestmentsInPrivatelyHeldCompanies" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/InvestmentsInPrivatelyHeldCompaniesDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/Leases" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/LeasesComponentsOfLeaseExpenseDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/LeasesDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_LesseeLeaseDescriptionLineItems" xlink:label="loc_us-gaap_LesseeLeaseDescriptionLineItems_627EDDE373E2042D774673A62477FB7B" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_LesseeLeaseDescriptionTable" xlink:label="loc_us-gaap_LesseeLeaseDescriptionTable_DB9C879F3659C8CF420C73A624767E24" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_627EDDE373E2042D774673A62477FB7B" xlink:to="loc_us-gaap_LesseeLeaseDescriptionTable_DB9C879F3659C8CF420C73A624767E24" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_RangeAxis" xlink:label="loc_srt_RangeAxis_DE95CC46B9BE615290F773A6247622B3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_LesseeLeaseDescriptionTable_DB9C879F3659C8CF420C73A624767E24" xlink:to="loc_srt_RangeAxis_DE95CC46B9BE615290F773A6247622B3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_99E8ED00C9E24B504C3F73A62476F148_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_RangeAxis_DE95CC46B9BE615290F773A6247622B3" xlink:to="loc_srt_RangeMember_99E8ED00C9E24B504C3F73A62476F148_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_RangeMember" xlink:label="loc_srt_RangeMember_99E8ED00C9E24B504C3F73A62476F148" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_RangeAxis_DE95CC46B9BE615290F773A6247622B3" xlink:to="loc_srt_RangeMember_99E8ED00C9E24B504C3F73A62476F148" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_MinimumMember" xlink:label="loc_srt_MinimumMember_0470EFCEB94933DBECF373A624767E83" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_99E8ED00C9E24B504C3F73A62476F148" xlink:to="loc_srt_MinimumMember_0470EFCEB94933DBECF373A624767E83" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_MaximumMember" xlink:label="loc_srt_MaximumMember_33DB7FB39C790537F1D673A6247713C4" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_RangeMember_99E8ED00C9E24B504C3F73A62476F148" xlink:to="loc_srt_MaximumMember_33DB7FB39C790537F1D673A6247713C4" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_OperatingLeaseRemainingLeaseTerm" xlink:label="loc_ftnt_OperatingLeaseRemainingLeaseTerm_1B26EA0E47D72759368973A62477F485" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_627EDDE373E2042D774673A62477FB7B" xlink:to="loc_ftnt_OperatingLeaseRemainingLeaseTerm_1B26EA0E47D72759368973A62477F485" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_LesseeOperatingLeaseRenewalTerm" xlink:label="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_065CAF4310EA836A730773A62478E052" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_627EDDE373E2042D774673A62477FB7B" xlink:to="loc_us-gaap_LesseeOperatingLeaseRenewalTerm_065CAF4310EA836A730773A62478E052" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OperatingLeasesRentExpenseNet" xlink:label="loc_us-gaap_OperatingLeasesRentExpenseNet_FBDD3BD17AAED47CD86073A62478DAD7" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_627EDDE373E2042D774673A62477FB7B" xlink:to="loc_us-gaap_OperatingLeasesRentExpenseNet_FBDD3BD17AAED47CD86073A62478DAD7" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_OperatingLeaseLeaseNotyetCommencedExpense" xlink:label="loc_ftnt_OperatingLeaseLeaseNotyetCommencedExpense_2DC513C7232DA9552C4573A62478209A" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_627EDDE373E2042D774673A62477FB7B" xlink:to="loc_ftnt_OperatingLeaseLeaseNotyetCommencedExpense_2DC513C7232DA9552C4573A62478209A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1" xlink:label="loc_us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1_7F8BD8151DA13328154873A62478270A" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_LesseeLeaseDescriptionLineItems_627EDDE373E2042D774673A62477FB7B" xlink:to="loc_us-gaap_LesseeOperatingLeaseLeaseNotYetCommencedTermOfContract1_7F8BD8151DA13328154873A62478270A" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/LeasesFuturePrincipalContractualObligationsForOperatingLeaseCommitmentsDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/LeasesMaturitiesOfOperatingLeaseLiabilitiesDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/LeasesSupplementalBalanceSheetInformationDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/LeasesSupplementalCashFlowInformationDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/LeasesTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/NetIncomePerShare" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/NetIncomePerShareAntiDilutiveSecuritiesDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_9892A3670880DD11AD4673A6223CC457" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_4B0BAD0FF189E41032D473A62239AE81" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_9892A3670880DD11AD4673A6223CC457" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_4B0BAD0FF189E41032D473A62239AE81" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_6369872C39559B99579273A6223907F7" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_4B0BAD0FF189E41032D473A62239AE81" xlink:to="loc_us-gaap_AwardTypeAxis_6369872C39559B99579273A6223907F7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ABD445C832B12BBF963273A6223A52AC_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_6369872C39559B99579273A6223907F7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ABD445C832B12BBF963273A6223A52AC_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ABD445C832B12BBF963273A6223A52AC" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_6369872C39559B99579273A6223907F7" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ABD445C832B12BBF963273A6223A52AC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_E3B449CCD67FD775178C73A6223A563B" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ABD445C832B12BBF963273A6223A52AC" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_E3B449CCD67FD775178C73A6223A563B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="loc_us-gaap_EmployeeStockOptionMember_9909D896394B0C9D8B9573A6223BF713" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ABD445C832B12BBF963273A6223A52AC" xlink:to="loc_us-gaap_EmployeeStockOptionMember_9909D896394B0C9D8B9573A6223BF713" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EmployeeStockMember" xlink:label="loc_us-gaap_EmployeeStockMember_1909546ADD0DDC120F6273A6223B8DCF" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_ABD445C832B12BBF963273A6223A52AC" xlink:to="loc_us-gaap_EmployeeStockMember_1909546ADD0DDC120F6273A6223B8DCF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_B3494DAC147FFFE471A773A6223B5030" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_4B0BAD0FF189E41032D473A62239AE81" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_B3494DAC147FFFE471A773A6223B5030" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_358F21B90F7BCD19F6BC73A6223BBF09_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_B3494DAC147FFFE471A773A6223B5030" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_358F21B90F7BCD19F6BC73A6223BBF09_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_358F21B90F7BCD19F6BC73A6223BBF09" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_B3494DAC147FFFE471A773A6223B5030" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_358F21B90F7BCD19F6BC73A6223BBF09" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StockCompensationPlanMember" xlink:label="loc_us-gaap_StockCompensationPlanMember_3A222668922765D7136F73A6223B1DDE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_358F21B90F7BCD19F6BC73A6223BBF09" xlink:to="loc_us-gaap_StockCompensationPlanMember_3A222668922765D7136F73A6223B1DDE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_036CA84ABF203F04452673A6223C1E84" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_9892A3670880DD11AD4673A6223CC457" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_036CA84ABF203F04452673A6223C1E84" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/NetIncomePerShareCalculationOfBasicAndDilutedDetails" xlink:type="extended">
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_EarningsPerShareLineItems" xlink:label="loc_ftnt_EarningsPerShareLineItems_083B85AF48F0DC8BCAF50370B0A8E696" xlink:type="locator" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_ScheduleOfEarningsPerShareTable" xlink:label="loc_ftnt_ScheduleOfEarningsPerShareTable_1B11218958271B34E46E0370B0A79DC5" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_ftnt_EarningsPerShareLineItems_083B85AF48F0DC8BCAF50370B0A8E696" xlink:to="loc_ftnt_ScheduleOfEarningsPerShareTable_1B11218958271B34E46E0370B0A79DC5" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_5F98076185249145D02E0370B0A7AAA3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ftnt_ScheduleOfEarningsPerShareTable_1B11218958271B34E46E0370B0A79DC5" xlink:to="loc_us-gaap_AwardTypeAxis_5F98076185249145D02E0370B0A7AAA3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_E428F2D49D5EDF376DA50370B0A77AA8_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_5F98076185249145D02E0370B0A7AAA3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_E428F2D49D5EDF376DA50370B0A77AA8_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_E428F2D49D5EDF376DA50370B0A77AA8" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_5F98076185249145D02E0370B0A7AAA3" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_E428F2D49D5EDF376DA50370B0A77AA8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_9A4E0B95AE3D7BDFC6340370B0A710DA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_E428F2D49D5EDF376DA50370B0A77AA8" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_9A4E0B95AE3D7BDFC6340370B0A710DA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="loc_us-gaap_EmployeeStockOptionMember_18343FF49297F7B9F1300370B0A844AC" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_E428F2D49D5EDF376DA50370B0A77AA8" xlink:to="loc_us-gaap_EmployeeStockOptionMember_18343FF49297F7B9F1300370B0A844AC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EmployeeStockMember" xlink:label="loc_us-gaap_EmployeeStockMember_69F82F85B65158DC76370370B0A821D0" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_E428F2D49D5EDF376DA50370B0A77AA8" xlink:to="loc_us-gaap_EmployeeStockMember_69F82F85B65158DC76370370B0A821D0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_8BAB54EEF188A6B52C9B0370B0A8A648" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_083B85AF48F0DC8BCAF50370B0A8E696" xlink:to="loc_us-gaap_NetIncomeLoss_8BAB54EEF188A6B52C9B0370B0A8A648" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasicAbstract" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasicAbstract_DB22B0E4A278282719890370B0A83846" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_083B85AF48F0DC8BCAF50370B0A8E696" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasicAbstract_DB22B0E4A278282719890370B0A83846" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_ACD39F266937F2A6AA9D0370B0A86C45" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasicAbstract_DB22B0E4A278282719890370B0A83846" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_ACD39F266937F2A6AA9D0370B0A86C45" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_F179F7DEE77776E28B150370B0A9D62F" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_083B85AF48F0DC8BCAF50370B0A8E696" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_F179F7DEE77776E28B150370B0A9D62F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract" xlink:label="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_A52E6D73ED263D1C69D60370B0A98A1A" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_083B85AF48F0DC8BCAF50370B0A8E696" xlink:to="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_A52E6D73ED263D1C69D60370B0A98A1A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements" xlink:label="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_3F7062C746D87BDBD7760370B0A95D3C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_A52E6D73ED263D1C69D60370B0A98A1A" xlink:to="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_3F7062C746D87BDBD7760370B0A95D3C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_BEF84DB1D23C182B4AED0370B0AAC214" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_083B85AF48F0DC8BCAF50370B0A8E696" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_BEF84DB1D23C182B4AED0370B0AAC214" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EarningsPerShareBasic" xlink:label="loc_us-gaap_EarningsPerShareBasic_0552BFF145D1BCE0B69D0370B0AA611D" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_083B85AF48F0DC8BCAF50370B0A8E696" xlink:to="loc_us-gaap_EarningsPerShareBasic_0552BFF145D1BCE0B69D0370B0AA611D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EarningsPerShareDiluted" xlink:label="loc_us-gaap_EarningsPerShareDiluted_B0069A9AD4B5F68CD3BA0370B0AA821A" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_083B85AF48F0DC8BCAF50370B0A8E696" xlink:to="loc_us-gaap_EarningsPerShareDiluted_B0069A9AD4B5F68CD3BA0370B0AA821A" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/NetIncomePerShareTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/PropertyAndEquipmentnet" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/PropertyAndEquipmentnetDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_17826D369C0C9BC21C6B73A624977386" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_6B9CF5F334DB46E776DF73A62494023E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_17826D369C0C9BC21C6B73A624977386" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_6B9CF5F334DB46E776DF73A62494023E" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_74FFA6B3EA9FBBCE494873A62495A00F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_6B9CF5F334DB46E776DF73A62494023E" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_74FFA6B3EA9FBBCE494873A62495A00F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_81AF8AEF81A45ED9B60373A62496050D_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_74FFA6B3EA9FBBCE494873A62495A00F" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_81AF8AEF81A45ED9B60373A62496050D_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_81AF8AEF81A45ED9B60373A62496050D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_74FFA6B3EA9FBBCE494873A62495A00F" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_81AF8AEF81A45ED9B60373A62496050D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_LandMember" xlink:label="loc_us-gaap_LandMember_7A1CBB5A9A48BD53B20073A624962A41" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_81AF8AEF81A45ED9B60373A62496050D" xlink:to="loc_us-gaap_LandMember_7A1CBB5A9A48BD53B20073A624962A41" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_BuildingAndBuildingImprovementsMember" xlink:label="loc_us-gaap_BuildingAndBuildingImprovementsMember_365235697B9F2020C84573A62496FE3B" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_81AF8AEF81A45ED9B60373A62496050D" xlink:to="loc_us-gaap_BuildingAndBuildingImprovementsMember_365235697B9F2020C84573A62496FE3B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ComputerEquipmentMember" xlink:label="loc_us-gaap_ComputerEquipmentMember_B34EA30F3B2C1C5D91E473A62496119A" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_81AF8AEF81A45ED9B60373A62496050D" xlink:to="loc_us-gaap_ComputerEquipmentMember_B34EA30F3B2C1C5D91E473A62496119A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_LeaseholdsAndLeaseholdImprovementsMember" xlink:label="loc_us-gaap_LeaseholdsAndLeaseholdImprovementsMember_97BE9B169F989CE8E4BC73A62497B7AB" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_81AF8AEF81A45ED9B60373A62496050D" xlink:to="loc_us-gaap_LeaseholdsAndLeaseholdImprovementsMember_97BE9B169F989CE8E4BC73A62497B7AB" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_EvaluationUnitsMember" xlink:label="loc_ftnt_EvaluationUnitsMember_87F5AF6342442BAB8FD373A624972BA8" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_81AF8AEF81A45ED9B60373A62496050D" xlink:to="loc_ftnt_EvaluationUnitsMember_87F5AF6342442BAB8FD373A624972BA8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_FurnitureAndFixturesMember" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_CAA45C64E126112A1F8E73A624973D8B" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_81AF8AEF81A45ED9B60373A62496050D" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_CAA45C64E126112A1F8E73A624973D8B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ConstructionInProgressMember" xlink:label="loc_us-gaap_ConstructionInProgressMember_BBD7E63C4834043927FB73A624976D0D" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_81AF8AEF81A45ED9B60373A62496050D" xlink:to="loc_us-gaap_ConstructionInProgressMember_BBD7E63C4834043927FB73A624976D0D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract_8BB645502CE15E7E8B0F73A62498D869" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_17826D369C0C9BC21C6B73A624977386" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract_8BB645502CE15E7E8B0F73A62498D869" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PropertyPlantAndEquipmentGross" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_C3B3DDF999750AC6075073A624985B63" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract_8BB645502CE15E7E8B0F73A62498D869" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_C3B3DDF999750AC6075073A624985B63" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_3EA93E605B49D96E1E7B73A624980A83" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract_8BB645502CE15E7E8B0F73A62498D869" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_3EA93E605B49D96E1E7B73A624980A83" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_AFCB03F476CCB29874BC73A6249F1C6F" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract_8BB645502CE15E7E8B0F73A62498D869" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_AFCB03F476CCB29874BC73A6249F1C6F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_Depreciation" xlink:label="loc_us-gaap_Depreciation_9F9834163A3A9356DBC573A6249FCAD2" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_17826D369C0C9BC21C6B73A624977386" xlink:to="loc_us-gaap_Depreciation_9F9834163A3A9356DBC573A6249FCAD2" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/PropertyAndEquipmentnetTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/RevenueRecognition" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/RevenueRecognitionDetails" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/RevenueRecognitionDisaggregatedRevenuesDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_DisaggregationOfRevenueLineItems" xlink:label="loc_us-gaap_DisaggregationOfRevenueLineItems_2694F267534C7D27867573A624370AD3" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_DisaggregationOfRevenueTable" xlink:label="loc_us-gaap_DisaggregationOfRevenueTable_3922709E45C44F9CCA3173A624344858" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_2694F267534C7D27867573A624370AD3" xlink:to="loc_us-gaap_DisaggregationOfRevenueTable_3922709E45C44F9CCA3173A624344858" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_ProductOrServiceAxis" xlink:label="loc_srt_ProductOrServiceAxis_DCCFF3EE733CB9E7533E73A624350E52" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DisaggregationOfRevenueTable_3922709E45C44F9CCA3173A624344858" xlink:to="loc_srt_ProductOrServiceAxis_DCCFF3EE733CB9E7533E73A624350E52" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_B3A1B9744289D9C547FD73A624352744_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_ProductOrServiceAxis_DCCFF3EE733CB9E7533E73A624350E52" xlink:to="loc_srt_ProductsAndServicesDomain_B3A1B9744289D9C547FD73A624352744_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_ProductsAndServicesDomain" xlink:label="loc_srt_ProductsAndServicesDomain_B3A1B9744289D9C547FD73A624352744" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_ProductOrServiceAxis_DCCFF3EE733CB9E7533E73A624350E52" xlink:to="loc_srt_ProductsAndServicesDomain_B3A1B9744289D9C547FD73A624352744" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ProductMember" xlink:label="loc_us-gaap_ProductMember_5C3B09A5FB0682EC04C173A62436B4BA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_B3A1B9744289D9C547FD73A624352744" xlink:to="loc_us-gaap_ProductMember_5C3B09A5FB0682EC04C173A62436B4BA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ServiceMember" xlink:label="loc_us-gaap_ServiceMember_DE27A1B28B08CFAEC5DE73A62436C7AD" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_ProductsAndServicesDomain_B3A1B9744289D9C547FD73A624352744" xlink:to="loc_us-gaap_ServiceMember_DE27A1B28B08CFAEC5DE73A62436C7AD" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_SecuritySubscriptionMember" xlink:label="loc_ftnt_SecuritySubscriptionMember_EE6CF9AE0D0A2A9E244073A6243680A7" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ServiceMember_DE27A1B28B08CFAEC5DE73A62436C7AD" xlink:to="loc_ftnt_SecuritySubscriptionMember_EE6CF9AE0D0A2A9E244073A6243680A7" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_TechnicalSupportandOtherMember" xlink:label="loc_ftnt_TechnicalSupportandOtherMember_3D53F40CBE7B4FFF0DCE73A6243737CF" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ServiceMember_DE27A1B28B08CFAEC5DE73A62436C7AD" xlink:to="loc_ftnt_TechnicalSupportandOtherMember_3D53F40CBE7B4FFF0DCE73A6243737CF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_B143F72706A56481F0DD73A624370A7E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DisaggregationOfRevenueLineItems_2694F267534C7D27867573A624370AD3" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_B143F72706A56481F0DD73A624370A7E" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/RevenueRecognitionPerformanceObligationSatisfactionPeriodDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_B3FD0E87504EED999A7E73A622AF3F14" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_EA52D5BA78103918C9F973A622AECC4D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_B3FD0E87504EED999A7E73A622AF3F14" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_EA52D5BA78103918C9F973A622AECC4D" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_82D0579CE385D63DDA5273A622AEF820" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionTable_EA52D5BA78103918C9F973A622AECC4D" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionStartDateAxis_82D0579CE385D63DDA5273A622AEF820" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1" xlink:label="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_BAB95B22045D6222236073A622AFD155" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionLineItems_B3FD0E87504EED999A7E73A622AF3F14" xlink:to="loc_us-gaap_RevenueRemainingPerformanceObligationExpectedTimingOfSatisfactionPeriod1_BAB95B22045D6222236073A622AFD155" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/RevenueRecognitionTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/SegmentInformation" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/SegmentInformationDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_0731D506D240463436B30370AF8EDFBC" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_9957DC043FDC7AAA3A020370AF89B238" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_0731D506D240463436B30370AF8EDFBC" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_9957DC043FDC7AAA3A020370AF89B238" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_StatementGeographicalAxis" xlink:label="loc_srt_StatementGeographicalAxis_0F3B915FD580EA48864A0370AF8A953A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_9957DC043FDC7AAA3A020370AF89B238" xlink:to="loc_srt_StatementGeographicalAxis_0F3B915FD580EA48864A0370AF8A953A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_SegmentGeographicalDomain" xlink:label="loc_srt_SegmentGeographicalDomain_5B3ACB4AB72E4FB3CB3F0370AF8A1073_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_StatementGeographicalAxis_0F3B915FD580EA48864A0370AF8A953A" xlink:to="loc_srt_SegmentGeographicalDomain_5B3ACB4AB72E4FB3CB3F0370AF8A1073_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_SegmentGeographicalDomain" xlink:label="loc_srt_SegmentGeographicalDomain_5B3ACB4AB72E4FB3CB3F0370AF8A1073" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_StatementGeographicalAxis_0F3B915FD580EA48864A0370AF8A953A" xlink:to="loc_srt_SegmentGeographicalDomain_5B3ACB4AB72E4FB3CB3F0370AF8A1073" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_AmericasMember" xlink:label="loc_srt_AmericasMember_1A700C0D58FD585B7D690370AF8A0F10" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_5B3ACB4AB72E4FB3CB3F0370AF8A1073" xlink:to="loc_srt_AmericasMember_1A700C0D58FD585B7D690370AF8A0F10" xlink:type="arc" />
    <link:loc xlink:href="https://xbrl.sec.gov/country/2017/country-2017-01-31.xsd#country_US" xlink:label="loc_country_US_A242D8FBEE167287EB550370AF8B16FA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_AmericasMember_1A700C0D58FD585B7D690370AF8A0F10" xlink:to="loc_country_US_A242D8FBEE167287EB550370AF8B16FA" xlink:type="arc" />
    <link:loc xlink:href="https://xbrl.sec.gov/country/2017/country-2017-01-31.xsd#country_CA" xlink:label="loc_country_CA_BFBA1959034764B46F2B0370AF8BFE6F" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_AmericasMember_1A700C0D58FD585B7D690370AF8A0F10" xlink:to="loc_country_CA_BFBA1959034764B46F2B0370AF8BFE6F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_LatinAmericaMember" xlink:label="loc_srt_LatinAmericaMember_8A83E0A14011E366C5790370AF8B5BCD" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_AmericasMember_1A700C0D58FD585B7D690370AF8A0F10" xlink:to="loc_srt_LatinAmericaMember_8A83E0A14011E366C5790370AF8B5BCD" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_OtherAmericasMember" xlink:label="loc_ftnt_OtherAmericasMember_0233D3A14E77C932A94B0370AF8B0C98" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_AmericasMember_1A700C0D58FD585B7D690370AF8A0F10" xlink:to="loc_ftnt_OtherAmericasMember_0233D3A14E77C932A94B0370AF8B0C98" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_EuropeMiddleEastAndAfricaMember" xlink:label="loc_ftnt_EuropeMiddleEastAndAfricaMember_4E42153E76D4047036BD0370AF8B548C" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_5B3ACB4AB72E4FB3CB3F0370AF8A1073" xlink:to="loc_ftnt_EuropeMiddleEastAndAfricaMember_4E42153E76D4047036BD0370AF8B548C" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_AsiaPacificAndJapanMember" xlink:label="loc_ftnt_AsiaPacificAndJapanMember_5D40718A1A106882E1B30370AF8C0553" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_SegmentGeographicalDomain_5B3ACB4AB72E4FB3CB3F0370AF8A1073" xlink:to="loc_ftnt_AsiaPacificAndJapanMember_5D40718A1A106882E1B30370AF8C0553" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_MajorCustomersAxis" xlink:label="loc_srt_MajorCustomersAxis_53E8794B642BE850BE070370AF8CCDC8" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_9957DC043FDC7AAA3A020370AF89B238" xlink:to="loc_srt_MajorCustomersAxis_53E8794B642BE850BE070370AF8CCDC8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_NameOfMajorCustomerDomain" xlink:label="loc_srt_NameOfMajorCustomerDomain_CD8F05D85DEA3D865F190370AF8C6526_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_srt_MajorCustomersAxis_53E8794B642BE850BE070370AF8CCDC8" xlink:to="loc_srt_NameOfMajorCustomerDomain_CD8F05D85DEA3D865F190370AF8C6526_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/srt/2019/elts/srt-2019-01-31.xsd#srt_NameOfMajorCustomerDomain" xlink:label="loc_srt_NameOfMajorCustomerDomain_CD8F05D85DEA3D865F190370AF8C6526" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_srt_MajorCustomersAxis_53E8794B642BE850BE070370AF8CCDC8" xlink:to="loc_srt_NameOfMajorCustomerDomain_CD8F05D85DEA3D865F190370AF8C6526" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_ExclusiveNetworksGroupMember" xlink:label="loc_ftnt_ExclusiveNetworksGroupMember_9102BB4763D83912DE7C0370AF8C4771" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_CD8F05D85DEA3D865F190370AF8C6526" xlink:to="loc_ftnt_ExclusiveNetworksGroupMember_9102BB4763D83912DE7C0370AF8C4771" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_IngramMicroMember" xlink:label="loc_ftnt_IngramMicroMember_5110C5362B604AB89B2F0370AF8CD278" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_srt_NameOfMajorCustomerDomain_CD8F05D85DEA3D865F190370AF8C6526" xlink:to="loc_ftnt_IngramMicroMember_5110C5362B604AB89B2F0370AF8CD278" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ConcentrationRiskByTypeAxis" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_558C0437C39527DE4D2A0370AF8D83A6" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_9957DC043FDC7AAA3A020370AF89B238" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_558C0437C39527DE4D2A0370AF8D83A6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ConcentrationRiskTypeDomain" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_DC44197F7F2C2CB61B760370AF8DB5DE_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_558C0437C39527DE4D2A0370AF8D83A6" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_DC44197F7F2C2CB61B760370AF8DB5DE_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ConcentrationRiskTypeDomain" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_DC44197F7F2C2CB61B760370AF8DB5DE" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_558C0437C39527DE4D2A0370AF8D83A6" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_DC44197F7F2C2CB61B760370AF8DB5DE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CustomerConcentrationRiskMember" xlink:label="loc_us-gaap_CustomerConcentrationRiskMember_3E78715CA7CCEDE37D0A0370AF8D83FA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_DC44197F7F2C2CB61B760370AF8DB5DE" xlink:to="loc_us-gaap_CustomerConcentrationRiskMember_3E78715CA7CCEDE37D0A0370AF8D83FA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_05583AD5B7CF781C42DD0370AF8D5E5E" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_9957DC043FDC7AAA3A020370AF89B238" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_05583AD5B7CF781C42DD0370AF8D5E5E" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ConcentrationRiskBenchmarkDomain" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_E532FB01CAF759B35AA50370AF8E5026_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_05583AD5B7CF781C42DD0370AF8D5E5E" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_E532FB01CAF759B35AA50370AF8E5026_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ConcentrationRiskBenchmarkDomain" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_E532FB01CAF759B35AA50370AF8E5026" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_05583AD5B7CF781C42DD0370AF8D5E5E" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_E532FB01CAF759B35AA50370AF8E5026" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SalesRevenueNetMember" xlink:label="loc_us-gaap_SalesRevenueNetMember_7D9193EF8509773320E50370AF8E4FB4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_E532FB01CAF759B35AA50370AF8E5026" xlink:to="loc_us-gaap_SalesRevenueNetMember_7D9193EF8509773320E50370AF8E4FB4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccountsReceivableMember" xlink:label="loc_us-gaap_AccountsReceivableMember_7407692DF73D15EADFC00370AF8E6B64" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_E532FB01CAF759B35AA50370AF8E5026" xlink:to="loc_us-gaap_AccountsReceivableMember_7407692DF73D15EADFC00370AF8E6B64" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_NumberOfBusinessActivities" xlink:label="loc_ftnt_NumberOfBusinessActivities_139C91AA754FD81B0BB40370AF8E740E" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_0731D506D240463436B30370AF8EDFBC" xlink:to="loc_ftnt_NumberOfBusinessActivities_139C91AA754FD81B0BB40370AF8E740E" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_NumberOfSegmentManagersResponsibleForOperations" xlink:label="loc_ftnt_NumberOfSegmentManagersResponsibleForOperations_02071007FB5CA35298C00370AF8F2C36" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_0731D506D240463436B30370AF8EDFBC" xlink:to="loc_ftnt_NumberOfSegmentManagersResponsibleForOperations_02071007FB5CA35298C00370AF8F2C36" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NumberOfOperatingSegments" xlink:label="loc_us-gaap_NumberOfOperatingSegments_703DDDA53AC726FAE6C50370AF8F2F21" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_0731D506D240463436B30370AF8EDFBC" xlink:to="loc_us-gaap_NumberOfOperatingSegments_703DDDA53AC726FAE6C50370AF8F2F21" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NumberOfReportableSegments" xlink:label="loc_us-gaap_NumberOfReportableSegments_2F2DEF4D64D86FF5E39B0370AF8FD464" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_0731D506D240463436B30370AF8EDFBC" xlink:to="loc_us-gaap_NumberOfReportableSegments_2F2DEF4D64D86FF5E39B0370AF8FD464" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax" xlink:label="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_9A789DC146CAA00816F20370AF8F8BDD" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_0731D506D240463436B30370AF8EDFBC" xlink:to="loc_us-gaap_RevenueFromContractWithCustomerExcludingAssessedTax_9A789DC146CAA00816F20370AF8F8BDD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_E3A710000E74C3667D8B0370AF8FDBD6" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_0731D506D240463436B30370AF8EDFBC" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_E3A710000E74C3667D8B0370AF8FDBD6" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ConcentrationRiskPercentage1" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_CA7EF5CF6AE1B6E837AC0370AF8F2AB0" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_0731D506D240463436B30370AF8EDFBC" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_CA7EF5CF6AE1B6E837AC0370AF8F2AB0" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/SegmentInformationTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/StockholdersEquity" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/StockholdersEquityAllocationOfStockBasedCompensationExpenseDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_B40696DE030EFDEA71540370B011F5FF" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_286F2DDEFE9D13C0A7680370B00F40DA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_B40696DE030EFDEA71540370B011F5FF" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_286F2DDEFE9D13C0A7680370B00F40DA" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_978335181EAEB9BF8A860370B00FCF7D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_286F2DDEFE9D13C0A7680370B00F40DA" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_978335181EAEB9BF8A860370B00FCF7D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_AE40EA64E8C89B7263680370B00F8065_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_978335181EAEB9BF8A860370B00FCF7D" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_AE40EA64E8C89B7263680370B00F8065_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_AE40EA64E8C89B7263680370B00F8065" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_978335181EAEB9BF8A860370B00FCF7D" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_AE40EA64E8C89B7263680370B00F8065" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_CostOfGoodsSoldMember" xlink:label="loc_ftnt_CostOfGoodsSoldMember_B6861E7261521454C4870370B01010D6" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_AE40EA64E8C89B7263680370B00F8065" xlink:to="loc_ftnt_CostOfGoodsSoldMember_B6861E7261521454C4870370B01010D6" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_CostOfServicesMember" xlink:label="loc_ftnt_CostOfServicesMember_EE6F09D4E9ED65B2D0300370B0100F8A" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_AE40EA64E8C89B7263680370B00F8065" xlink:to="loc_ftnt_CostOfServicesMember_EE6F09D4E9ED65B2D0300370B0100F8A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ResearchAndDevelopmentExpenseMember" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_607BF8782F3B081327770370B010814A" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_AE40EA64E8C89B7263680370B00F8065" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_607BF8782F3B081327770370B010814A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SellingAndMarketingExpenseMember" xlink:label="loc_us-gaap_SellingAndMarketingExpenseMember_8F7266C245BC99BA7F490370B010C006" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_AE40EA64E8C89B7263680370B00F8065" xlink:to="loc_us-gaap_SellingAndMarketingExpenseMember_8F7266C245BC99BA7F490370B010C006" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_GeneralAndAdministrativeExpenseMember" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpenseMember_ED54ACDA4FCCB5AD39710370B010AC73" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_AE40EA64E8C89B7263680370B00F8065" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpenseMember_ED54ACDA4FCCB5AD39710370B010AC73" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_80DB0737F23D714A4C190370B0100180" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_286F2DDEFE9D13C0A7680370B00F40DA" xlink:to="loc_us-gaap_AwardTypeAxis_80DB0737F23D714A4C190370B0100180" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3A01A2CB9DB191CE2EBD0370B011D0DD_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_80DB0737F23D714A4C190370B0100180" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3A01A2CB9DB191CE2EBD0370B011D0DD_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3A01A2CB9DB191CE2EBD0370B011D0DD" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_80DB0737F23D714A4C190370B0100180" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3A01A2CB9DB191CE2EBD0370B011D0DD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_18F8A4B26E71036E32840370B011D2CD" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3A01A2CB9DB191CE2EBD0370B011D0DD" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_18F8A4B26E71036E32840370B011D2CD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="loc_us-gaap_EmployeeStockOptionMember_A78EF0D1718B5E2EB8120370B011327F" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3A01A2CB9DB191CE2EBD0370B011D0DD" xlink:to="loc_us-gaap_EmployeeStockOptionMember_A78EF0D1718B5E2EB8120370B011327F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EmployeeStockMember" xlink:label="loc_us-gaap_EmployeeStockMember_9D3134F260E4EA533C7B0370B01126FA" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_3A01A2CB9DB191CE2EBD0370B011D0DD" xlink:to="loc_us-gaap_EmployeeStockMember_9D3134F260E4EA533C7B0370B01126FA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_924D214E333A1C0238600370B0122579" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_B40696DE030EFDEA71540370B011F5FF" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_924D214E333A1C0238600370B0122579" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions_A3A5802D47578FAE2EF70370B0120A1D" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_B40696DE030EFDEA71540370B011F5FF" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromExerciseOfStockOptions_A3A5802D47578FAE2EF70370B0120A1D" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/StockholdersEquityDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_22DC691945F71A3F0BE10370B028B533" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_C94B35D5B334263B8A850370B026E391" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_22DC691945F71A3F0BE10370B028B533" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_C94B35D5B334263B8A850370B026E391" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_7137EBF92DA74EA6D2DD0370B0279DC0" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_C94B35D5B334263B8A850370B026E391" xlink:to="loc_us-gaap_AwardTypeAxis_7137EBF92DA74EA6D2DD0370B0279DC0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_9199ACA8D0777AD941900370B028896A_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_7137EBF92DA74EA6D2DD0370B0279DC0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_9199ACA8D0777AD941900370B028896A_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_9199ACA8D0777AD941900370B028896A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_7137EBF92DA74EA6D2DD0370B0279DC0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_9199ACA8D0777AD941900370B028896A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StockCompensationPlanMember" xlink:label="loc_us-gaap_StockCompensationPlanMember_6D6601C004C0E044D6520370B0281C26" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_9199ACA8D0777AD941900370B028896A" xlink:to="loc_us-gaap_StockCompensationPlanMember_6D6601C004C0E044D6520370B0281C26" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="loc_us-gaap_EmployeeStockOptionMember_AF1F6847EA319A2BC8940370B02811AD" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_9199ACA8D0777AD941900370B028896A" xlink:to="loc_us-gaap_EmployeeStockOptionMember_AF1F6847EA319A2BC8940370B02811AD" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberofSharesDeregistered" xlink:label="loc_ftnt_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberofSharesDeregistered_5C9A6FF2CED8732789440370B0288C1C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_22DC691945F71A3F0BE10370B028B533" xlink:to="loc_ftnt_SharebasedCompensationArrangementbySharebasedPaymentAwardNumberofSharesDeregistered_5C9A6FF2CED8732789440370B0288C1C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_45207DB9B72736A422950370B029E5AA" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_22DC691945F71A3F0BE10370B028B533" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardNumberOfSharesAvailableForGrant_45207DB9B72736A422950370B029E5AA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_FCE31DC1FC19F1AFC8740370B0290927" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_22DC691945F71A3F0BE10370B028B533" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_FCE31DC1FC19F1AFC8740370B0290927" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_B9802B9A7F72307A56F90370B0291F8A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_FCE31DC1FC19F1AFC8740370B0290927" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_B9802B9A7F72307A56F90370B0291F8A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_519C1512F15F6F1186160370B02928E3" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_FCE31DC1FC19F1AFC8740370B0290927" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_519C1512F15F6F1186160370B02928E3" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_5FFFFD9CDEA16AFD77750370B029FDAF" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_FCE31DC1FC19F1AFC8740370B0290927" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_5FFFFD9CDEA16AFD77750370B029FDAF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_3B540FE7A720B0A81ECB0370B02ADDE8" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_FCE31DC1FC19F1AFC8740370B0290927" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_3B540FE7A720B0A81ECB0370B02ADDE8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_3BD4A5D9ADDDF56AC8B30370B02A5E8B" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_22DC691945F71A3F0BE10370B028B533" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_3BD4A5D9ADDDF56AC8B30370B02A5E8B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_A0E76DCEAFE322C55F8D0370B02A3910" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_3BD4A5D9ADDDF56AC8B30370B02A5E8B" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_A0E76DCEAFE322C55F8D0370B02A3910" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_AF91808FEB27B143932B0370B02A3539" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_3BD4A5D9ADDDF56AC8B30370B02A5E8B" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_AF91808FEB27B143932B0370B02A3539" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_33C8BD28D699D8960E910370B02A5B60" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_3BD4A5D9ADDDF56AC8B30370B02A5E8B" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_33C8BD28D699D8960E910370B02A5B60" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_770A33322D8983434FA70370B02B4144" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_3BD4A5D9ADDDF56AC8B30370B02A5E8B" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_770A33322D8983434FA70370B02B4144" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_93C1BEC29311F0EA5D5D0370B02BEC77" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_22DC691945F71A3F0BE10370B028B533" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_93C1BEC29311F0EA5D5D0370B02BEC77" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_BB30A979581D1C7194DE0370B02B4F14" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_93C1BEC29311F0EA5D5D0370B02BEC77" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_BB30A979581D1C7194DE0370B02B4F14" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_0B33834D6D09D97AFF1E0370B02BC591" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_93C1BEC29311F0EA5D5D0370B02BEC77" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_0B33834D6D09D97AFF1E0370B02BC591" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_33034401BA798F4CE5F40370B02BDD52" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_93C1BEC29311F0EA5D5D0370B02BEC77" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_33034401BA798F4CE5F40370B02BDD52" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_D12DA94E2FB411AB34EE0370B02C9017" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_93C1BEC29311F0EA5D5D0370B02BEC77" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_D12DA94E2FB411AB34EE0370B02C9017" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_A81B4143E64FF8D2E54F0370B02C763B" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_22DC691945F71A3F0BE10370B028B533" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_A81B4143E64FF8D2E54F0370B02C763B" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_340B6A483B047C77CB740370B02C8099" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_A81B4143E64FF8D2E54F0370B02C763B" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingNumber_340B6A483B047C77CB740370B02C8099" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_F51A0A3B8498B54591470370B02C7B96" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_A81B4143E64FF8D2E54F0370B02C763B" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageExercisePrice_F51A0A3B8498B54591470370B02C7B96" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_5C467A4560F0A9742D180370B02D9B5F" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_A81B4143E64FF8D2E54F0370B02C763B" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingWeightedAverageRemainingContractualTerm1_5C467A4560F0A9742D180370B02D9B5F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_F80D1F7647EA4C14ACED0370B02D713F" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_A81B4143E64FF8D2E54F0370B02C763B" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsVestedAndExpectedToVestOutstandingAggregateIntrinsicValue_F80D1F7647EA4C14ACED0370B02D713F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_6B2354CB2B45B4420EF80370B02DEDEF" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_A81B4143E64FF8D2E54F0370B02C763B" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_6B2354CB2B45B4420EF80370B02DEDEF" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_591627548ED6E3956E3A0370B02D3155" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_A81B4143E64FF8D2E54F0370B02C763B" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_591627548ED6E3956E3A0370B02D3155" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_096730C3BE11BBFA37DE0370B02D4FB2" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_A81B4143E64FF8D2E54F0370B02C763B" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_096730C3BE11BBFA37DE0370B02D4FB2" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_D4B0512BF57F778E8C070370B02D7C4D" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_A81B4143E64FF8D2E54F0370B02C763B" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_D4B0512BF57F778E8C070370B02D7C4D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_C3CE4A893E3D162634000370B02E3803" xlink:type="locator" />
    <link:definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_22DC691945F71A3F0BE10370B028B533" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_C3CE4A893E3D162634000370B02E3803" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_CB6CC58B456A0215F6F80370B02E4C70" xlink:type="locator" />
    <link:definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_22DC691945F71A3F0BE10370B028B533" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_CB6CC58B456A0215F6F80370B02E4C70" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_CF7569B16F2342DFD4370370B02EE029" xlink:type="locator" />
    <link:definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_22DC691945F71A3F0BE10370B028B533" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_CF7569B16F2342DFD4370370B02EE029" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_0A84A3B9169F4A8171900370B02EB7DE" xlink:type="locator" />
    <link:definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_22DC691945F71A3F0BE10370B028B533" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_0A84A3B9169F4A8171900370B02EB7DE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_111C3975DC2FCBFF49430370B02EE62C" xlink:type="locator" />
    <link:definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_22DC691945F71A3F0BE10370B028B533" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_111C3975DC2FCBFF49430370B02EE62C" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/StockholdersEquityRestrictedStockUnitsActivityDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_999B5BC077DADB19146404027B4C5555" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_2C766B104677071131B404027B4BC5AF" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_999B5BC077DADB19146404027B4C5555" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_2C766B104677071131B404027B4BC5AF" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_1EDDD2C8BAD60908D06D04027B4B3A4C" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_2C766B104677071131B404027B4BC5AF" xlink:to="loc_us-gaap_AwardTypeAxis_1EDDD2C8BAD60908D06D04027B4B3A4C" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_168A5964E5B6E160596E04027B4C3A95_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_1EDDD2C8BAD60908D06D04027B4B3A4C" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_168A5964E5B6E160596E04027B4C3A95_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_168A5964E5B6E160596E04027B4C3A95" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_1EDDD2C8BAD60908D06D04027B4B3A4C" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_168A5964E5B6E160596E04027B4C3A95" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_B713BCB4E37C8C91171B04027B4C96F7" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_168A5964E5B6E160596E04027B4C3A95" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_B713BCB4E37C8C91171B04027B4C96F7" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_BDC310B563F9E6B1193504027B4C149F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_999B5BC077DADB19146404027B4C5555" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_BDC310B563F9E6B1193504027B4C149F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_EEB3A5CA41BB7EEBF1C104027B4D9725" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_BDC310B563F9E6B1193504027B4C149F" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_EEB3A5CA41BB7EEBF1C104027B4D9725" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_9D270A34DE375CAB07FE04027B4D4257" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_BDC310B563F9E6B1193504027B4C149F" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_9D270A34DE375CAB07FE04027B4D4257" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_8967B80658F79BAB303204027B4D2859" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_BDC310B563F9E6B1193504027B4C149F" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_8967B80658F79BAB303204027B4D2859" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_C4F1BF0A89D50725890204027B4D945A" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_BDC310B563F9E6B1193504027B4C149F" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_C4F1BF0A89D50725890204027B4D945A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_714236DB2F19CFC055E804027B4E5320" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_999B5BC077DADB19146404027B4C5555" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_714236DB2F19CFC055E804027B4E5320" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_A445B918034947DB2B7404027B4E864A" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_714236DB2F19CFC055E804027B4E5320" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_A445B918034947DB2B7404027B4E864A" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_4D96187CB706BF08177F04027B4E92DA" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_714236DB2F19CFC055E804027B4E5320" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_4D96187CB706BF08177F04027B4E92DA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_09664F2D758C089C5B8104027B4E27ED" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_714236DB2F19CFC055E804027B4E5320" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_09664F2D758C089C5B8104027B4E27ED" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_CD603BD45CEDDD0B46E304027B4F1F6F" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_714236DB2F19CFC055E804027B4E5320" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_CD603BD45CEDDD0B46E304027B4F1F6F" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_28D1A82C3936EF446F5D04027B4F9F92" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_999B5BC077DADB19146404027B4C5555" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_28D1A82C3936EF446F5D04027B4F9F92" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_DAFDFE4E0C57E07E103A04027B4FAF91" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_999B5BC077DADB19146404027B4C5555" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_DAFDFE4E0C57E07E103A04027B4FAF91" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation" xlink:label="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_4FEDDE6369BD40EE4E4404027B4F9125" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_999B5BC077DADB19146404027B4C5555" xlink:to="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_4FEDDE6369BD40EE4E4404027B4F9125" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation" xlink:label="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_050E4FDC1913F9885980040FEF9A89FD" xlink:type="locator" />
    <link:definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_999B5BC077DADB19146404027B4C5555" xlink:to="loc_us-gaap_AdjustmentsRelatedToTaxWithholdingForShareBasedCompensation_050E4FDC1913F9885980040FEF9A89FD" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/StockholdersEquityShareRepurchaseProgramDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_DB99BC2B39C340B2FE1D03CE286C9776" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_BEC9B4D4A81318B88FD403CE286BC8F3" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_DB99BC2B39C340B2FE1D03CE286C9776" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_BEC9B4D4A81318B88FD403CE286BC8F3" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareRepurchaseProgramAxis" xlink:label="loc_us-gaap_ShareRepurchaseProgramAxis_AA8F47572B0D9B6BD9B003CE286CE4AB" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_BEC9B4D4A81318B88FD403CE286BC8F3" xlink:to="loc_us-gaap_ShareRepurchaseProgramAxis_AA8F47572B0D9B6BD9B003CE286CE4AB" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_D2A7CA18C7BB19ADDE2A03CE286C6AED_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_AA8F47572B0D9B6BD9B003CE286CE4AB" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_D2A7CA18C7BB19ADDE2A03CE286C6AED_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_D2A7CA18C7BB19ADDE2A03CE286C6AED" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_AA8F47572B0D9B6BD9B003CE286CE4AB" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_D2A7CA18C7BB19ADDE2A03CE286C6AED" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_ShareRepurchaseProgramMember" xlink:label="loc_ftnt_ShareRepurchaseProgramMember_D0850CD0CD9C9A2DDA4703CE286CA628" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareRepurchaseProgramDomain_D2A7CA18C7BB19ADDE2A03CE286C6AED" xlink:to="loc_ftnt_ShareRepurchaseProgramMember_D0850CD0CD9C9A2DDA4703CE286CA628" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StockRepurchaseProgramAuthorizedAmount1" xlink:label="loc_us-gaap_StockRepurchaseProgramAuthorizedAmount1_D0DD39D599299E6FC49A03CE286DBF8D" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_DB99BC2B39C340B2FE1D03CE286C9776" xlink:to="loc_us-gaap_StockRepurchaseProgramAuthorizedAmount1_D0DD39D599299E6FC49A03CE286DBF8D" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_0CE10E1029A32D439E4603CE286DFDCC" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_DB99BC2B39C340B2FE1D03CE286C9776" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_0CE10E1029A32D439E4603CE286DFDCC" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_TreasuryStockAcquiredAverageCostPerShare" xlink:label="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_8756B55012F1BA3C7B7103CE286DA952" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_DB99BC2B39C340B2FE1D03CE286C9776" xlink:to="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_8756B55012F1BA3C7B7103CE286DA952" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_94339C59F86186FC950003CE286D6A26" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_DB99BC2B39C340B2FE1D03CE286C9776" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_94339C59F86186FC950003CE286D6A26" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1" xlink:label="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_3741A9926914D691C56E03CE286D96FB" xlink:type="locator" />
    <link:definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_DB99BC2B39C340B2FE1D03CE286C9776" xlink:to="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_3741A9926914D691C56E03CE286D96FB" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/StockholdersEquityTables" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/SubsequentEvents" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/SubsequentEventsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SubsequentEventLineItems" xlink:label="loc_us-gaap_SubsequentEventLineItems_5AB10FD837AA5C375F94192C66F65EF5" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SubsequentEventTable" xlink:label="loc_us-gaap_SubsequentEventTable_DFFE4894F19E5042A9C6192C66ED1C32" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_5AB10FD837AA5C375F94192C66F65EF5" xlink:to="loc_us-gaap_SubsequentEventTable_DFFE4894F19E5042A9C6192C66ED1C32" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_D644012A6AF693A06041192C66EEDE99" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_DFFE4894F19E5042A9C6192C66ED1C32" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_D644012A6AF693A06041192C66EEDE99" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_76BEB590FD9DE1458E61192C66EE8951_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_D644012A6AF693A06041192C66EEDE99" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_76BEB590FD9DE1458E61192C66EE8951_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_76BEB590FD9DE1458E61192C66EE8951" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_D644012A6AF693A06041192C66EEDE99" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_76BEB590FD9DE1458E61192C66EE8951" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_SubsequentEventMember" xlink:label="loc_us-gaap_SubsequentEventMember_8A67346A4951B281B3A9192C66EF82BD" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_76BEB590FD9DE1458E61192C66EE8951" xlink:to="loc_us-gaap_SubsequentEventMember_8A67346A4951B281B3A9192C66EF82BD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_F81B5F58AA8F0B271242192DEBA52137" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_DFFE4894F19E5042A9C6192C66ED1C32" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_F81B5F58AA8F0B271242192DEBA52137" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_73A2B3A7F854CE6FAADB192DEBA9BF10_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_F81B5F58AA8F0B271242192DEBA52137" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_73A2B3A7F854CE6FAADB192DEBA9BF10_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_73A2B3A7F854CE6FAADB192DEBA9BF10" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_F81B5F58AA8F0B271242192DEBA52137" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_73A2B3A7F854CE6FAADB192DEBA9BF10" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_EnSiloLtd.Member" xlink:label="loc_ftnt_EnSiloLtd.Member_16A5B784438BA7D7E2C1192E8033ED98" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_73A2B3A7F854CE6FAADB192DEBA9BF10" xlink:to="loc_ftnt_EnSiloLtd.Member_16A5B784438BA7D7E2C1192E8033ED98" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareRepurchaseProgramAxis" xlink:label="loc_us-gaap_ShareRepurchaseProgramAxis_114867330B3966B6F2EE192C66F09616" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_DFFE4894F19E5042A9C6192C66ED1C32" xlink:to="loc_us-gaap_ShareRepurchaseProgramAxis_114867330B3966B6F2EE192C66F09616" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_75995F7E295F13731C0E192C66F6FB0F_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_114867330B3966B6F2EE192C66F09616" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_75995F7E295F13731C0E192C66F6FB0F_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_75995F7E295F13731C0E192C66F6FB0F" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_114867330B3966B6F2EE192C66F09616" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_75995F7E295F13731C0E192C66F6FB0F" xlink:type="arc" />
    <link:loc xlink:href="ftnt-20190930.xsd#ftnt_ShareRepurchaseProgramMember" xlink:label="loc_ftnt_ShareRepurchaseProgramMember_644E2F3E221E475A98C7192C66F6B417" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareRepurchaseProgramDomain_75995F7E295F13731C0E192C66F6FB0F" xlink:to="loc_ftnt_ShareRepurchaseProgramMember_644E2F3E221E475A98C7192C66F6B417" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferred1" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_6646ACBB3D0E5FDACD39192D0E798907" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_5AB10FD837AA5C375F94192C66F65EF5" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_6646ACBB3D0E5FDACD39192D0E798907" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_C0FEA72C33D36B5A939C192C66F79BD8" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_5AB10FD837AA5C375F94192C66F65EF5" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_C0FEA72C33D36B5A939C192C66F79BD8" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_A321891577CB15984630192C66F767D0" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_5AB10FD837AA5C375F94192C66F65EF5" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_A321891577CB15984630192C66F767D0" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_TreasuryStockAcquiredAverageCostPerShare" xlink:label="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_C3D0AD123D7E77A2CF81192C66F7098D" xlink:type="locator" />
    <link:definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_5AB10FD837AA5C375F94192C66F65EF5" xlink:to="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_C3D0AD123D7E77A2CF81192C66F7098D" xlink:type="arc" />
  </link:definitionLink>
  <link:definitionLink xlink:role="http://www.fortinet.com/role/SummaryOfSignificantAccountingPolicies" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/SummaryOfSignificantAccountingPoliciesPolicies" xlink:type="extended" />
  <link:definitionLink xlink:role="http://www.fortinet.com/role/SummaryOfSignificantAccountingPoliciesRecentAccountingPronouncementsDetails" xlink:type="extended">
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NewAccountingPronouncementEarlyAdoptionLineItems" xlink:label="loc_us-gaap_NewAccountingPronouncementEarlyAdoptionLineItems_060F999E248C81A20BC20370B14E51A9" xlink:type="locator" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_NewAccountingPronouncementEarlyAdoptionTable" xlink:label="loc_us-gaap_NewAccountingPronouncementEarlyAdoptionTable_9D8904FEF4CFACB4686E0370B14C71B7" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_NewAccountingPronouncementEarlyAdoptionLineItems_060F999E248C81A20BC20370B14E51A9" xlink:to="loc_us-gaap_NewAccountingPronouncementEarlyAdoptionTable_9D8904FEF4CFACB4686E0370B14C71B7" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_7E786EF3EE58003E0AA20370B14C0E60" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_NewAccountingPronouncementEarlyAdoptionTable_9D8904FEF4CFACB4686E0370B14C71B7" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_7E786EF3EE58003E0AA20370B14C0E60" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_00C6D14B9495A03B323C0370B14C55CD_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_7E786EF3EE58003E0AA20370B14C0E60" xlink:to="loc_us-gaap_EquityComponentDomain_00C6D14B9495A03B323C0370B14C55CD_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_00C6D14B9495A03B323C0370B14C55CD" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_7E786EF3EE58003E0AA20370B14C0E60" xlink:to="loc_us-gaap_EquityComponentDomain_00C6D14B9495A03B323C0370B14C55CD" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeMember" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_64A39B1358A301866D8C0370B14C00E4" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_00C6D14B9495A03B323C0370B14C55CD" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeMember_64A39B1358A301866D8C0370B14C00E4" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_RetainedEarningsMember" xlink:label="loc_us-gaap_RetainedEarningsMember_2D962C8FE212DA7315160370B14DBD21" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_00C6D14B9495A03B323C0370B14C55CD" xlink:to="loc_us-gaap_RetainedEarningsMember_2D962C8FE212DA7315160370B14DBD21" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AdjustmentsForNewAccountingPronouncementsAxis" xlink:label="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_DF2B0DDE0FE98D4291060370B14DFC51" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_NewAccountingPronouncementEarlyAdoptionTable_9D8904FEF4CFACB4686E0370B14C71B7" xlink:to="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_DF2B0DDE0FE98D4291060370B14DFC51" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="loc_us-gaap_TypeOfAdoptionMember_11319BA1E8299919C2070370B14D08EA_default" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_DF2B0DDE0FE98D4291060370B14DFC51" xlink:to="loc_us-gaap_TypeOfAdoptionMember_11319BA1E8299919C2070370B14D08EA_default" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_TypeOfAdoptionMember" xlink:label="loc_us-gaap_TypeOfAdoptionMember_11319BA1E8299919C2070370B14D08EA" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AdjustmentsForNewAccountingPronouncementsAxis_DF2B0DDE0FE98D4291060370B14DFC51" xlink:to="loc_us-gaap_TypeOfAdoptionMember_11319BA1E8299919C2070370B14D08EA" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccountingStandardsUpdate201602Member" xlink:label="loc_us-gaap_AccountingStandardsUpdate201602Member_794566D28C9B00940A7A0370B14D3C19" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfAdoptionMember_11319BA1E8299919C2070370B14D08EA" xlink:to="loc_us-gaap_AccountingStandardsUpdate201602Member_794566D28C9B00940A7A0370B14D3C19" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_AccountingStandardsUpdate201802Member" xlink:label="loc_us-gaap_AccountingStandardsUpdate201802Member_FF96A25D5CA78BC73CBA0370B14E44EE" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfAdoptionMember_11319BA1E8299919C2070370B14D08EA" xlink:to="loc_us-gaap_AccountingStandardsUpdate201802Member_FF96A25D5CA78BC73CBA0370B14E44EE" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OperatingLeaseRightOfUseAsset" xlink:label="loc_us-gaap_OperatingLeaseRightOfUseAsset_06DC7A10A44CE0180A750370B14E7A98" xlink:type="locator" />
    <link:definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementEarlyAdoptionLineItems_060F999E248C81A20BC20370B14E51A9" xlink:to="loc_us-gaap_OperatingLeaseRightOfUseAsset_06DC7A10A44CE0180A750370B14E7A98" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_OperatingLeaseLiability" xlink:label="loc_us-gaap_OperatingLeaseLiability_3A3B85BA5B623F230F760370B14EC097" xlink:type="locator" />
    <link:definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementEarlyAdoptionLineItems_060F999E248C81A20BC20370B14E51A9" xlink:to="loc_us-gaap_OperatingLeaseLiability_3A3B85BA5B623F230F760370B14EC097" xlink:type="arc" />
    <link:loc xlink:href="http://xbrl.fasb.org/us-gaap/2019/elts/us-gaap-2019-01-31.xsd#us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption" xlink:label="loc_us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption_19490454458EE003B7F00370B14E85EA" xlink:type="locator" />
    <link:definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementEarlyAdoptionLineItems_060F999E248C81A20BC20370B14E51A9" xlink:to="loc_us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption_19490454458EE003B7F00370B14E85EA" xlink:type="arc" />
  </link:definitionLink>
</link:linkbase>
