<?xml version="1.0" encoding="US-ASCII"?>
<!--XBRL Document Created with Wdesk from Workiva-->
<!-- -->
<linkbase xsi:schemaLocation="http://www.xbrl.org/2003/linkbase http://www.xbrl.org/2003/xbrl-linkbase-2003-12-31.xsd" xmlns="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">
  <roleRef roleURI="http://www.fortinet.com/role/AccumulatedOtherComprehensiveIncomeLoss" xlink:href="ftnt-20160930.xsd#AccumulatedOtherComprehensiveIncomeLoss" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/AccumulatedOtherComprehensiveIncomeLossChangesInAccumulatedBalancesOfOtherComprehensiveIncomedetails" xlink:href="ftnt-20160930.xsd#AccumulatedOtherComprehensiveIncomeLossChangesInAccumulatedBalancesOfOtherComprehensiveIncomedetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/AccumulatedOtherComprehensiveIncomeLossReclassificationOutOfAccumulatedOtherComprehensiveIncomeDetails" xlink:href="ftnt-20160930.xsd#AccumulatedOtherComprehensiveIncomeLossReclassificationOutOfAccumulatedOtherComprehensiveIncomeDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/AccumulatedOtherComprehensiveIncomeLossTables" xlink:href="ftnt-20160930.xsd#AccumulatedOtherComprehensiveIncomeLossTables" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/BusinessCombinations" xlink:href="ftnt-20160930.xsd#BusinessCombinations" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/BusinessCombinationsAdditionalInformationDetails" xlink:href="ftnt-20160930.xsd#BusinessCombinationsAdditionalInformationDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/BusinessCombinationsIntangibleAssetsAcquiredDetails" xlink:href="ftnt-20160930.xsd#BusinessCombinationsIntangibleAssetsAcquiredDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/BusinessCombinationsPurchasePriceAllocationsDetails" xlink:href="ftnt-20160930.xsd#BusinessCombinationsPurchasePriceAllocationsDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/BusinessCombinationsTables" xlink:href="ftnt-20160930.xsd#BusinessCombinationsTables" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/CommitmentsAndContingencies" xlink:href="ftnt-20160930.xsd#CommitmentsAndContingencies" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/CommitmentsAndContingenciesDetails" xlink:href="ftnt-20160930.xsd#CommitmentsAndContingenciesDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/CommitmentsAndContingenciesMinimumOperatingLeasePaymentsDetails" xlink:href="ftnt-20160930.xsd#CommitmentsAndContingenciesMinimumOperatingLeasePaymentsDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/CommitmentsAndContingenciesTables" xlink:href="ftnt-20160930.xsd#CommitmentsAndContingenciesTables" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/CommitmentsAndContingenciesWarrantiesDetails" xlink:href="ftnt-20160930.xsd#CommitmentsAndContingenciesWarrantiesDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/CondensedConsolidatedBalanceSheetsUnaudited" xlink:href="ftnt-20160930.xsd#CondensedConsolidatedBalanceSheetsUnaudited" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/CondensedConsolidatedBalanceSheetsUnauditedParenthetical" xlink:href="ftnt-20160930.xsd#CondensedConsolidatedBalanceSheetsUnauditedParenthetical" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/CondensedConsolidatedStatementsOfCashFlowsUnaudited" xlink:href="ftnt-20160930.xsd#CondensedConsolidatedStatementsOfCashFlowsUnaudited" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/CondensedConsolidatedStatementsOfComprehensiveIncomeUnaudited" xlink:href="ftnt-20160930.xsd#CondensedConsolidatedStatementsOfComprehensiveIncomeUnaudited" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/CondensedConsolidatedStatementsOfOperationsUnaudited" xlink:href="ftnt-20160930.xsd#CondensedConsolidatedStatementsOfOperationsUnaudited" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/DefinedContributionPlans" xlink:href="ftnt-20160930.xsd#DefinedContributionPlans" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/DefinedContributionPlansDetails" xlink:href="ftnt-20160930.xsd#DefinedContributionPlansDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/DocumentAndEntityInformation" xlink:href="ftnt-20160930.xsd#DocumentAndEntityInformation" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/FinancialInstrumentsAndFairValue" xlink:href="ftnt-20160930.xsd#FinancialInstrumentsAndFairValue" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/FinancialInstrumentsAndFairValueFairValueMeasurementsDetails" xlink:href="ftnt-20160930.xsd#FinancialInstrumentsAndFairValueFairValueMeasurementsDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/FinancialInstrumentsAndFairValueInvestmentsDetails" xlink:href="ftnt-20160930.xsd#FinancialInstrumentsAndFairValueInvestmentsDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/FinancialInstrumentsAndFairValueTables" xlink:href="ftnt-20160930.xsd#FinancialInstrumentsAndFairValueTables" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/ForeignCurrencyDerivatives" xlink:href="ftnt-20160930.xsd#ForeignCurrencyDerivatives" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/ForeignCurrencyDerivativesDetails" xlink:href="ftnt-20160930.xsd#ForeignCurrencyDerivativesDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/ForeignCurrencyDerivativesTables" xlink:href="ftnt-20160930.xsd#ForeignCurrencyDerivativesTables" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/GoodwillAndOtherIntangibleAssetsNet" xlink:href="ftnt-20160930.xsd#GoodwillAndOtherIntangibleAssetsNet" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/GoodwillAndOtherIntangibleAssetsNetChangesInCarryingAmountOfGoodwillDetails" xlink:href="ftnt-20160930.xsd#GoodwillAndOtherIntangibleAssetsNetChangesInCarryingAmountOfGoodwillDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/GoodwillAndOtherIntangibleAssetsNetDetails" xlink:href="ftnt-20160930.xsd#GoodwillAndOtherIntangibleAssetsNetDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/GoodwillAndOtherIntangibleAssetsNetEstimatedFutureAmortizationExpenseDetails" xlink:href="ftnt-20160930.xsd#GoodwillAndOtherIntangibleAssetsNetEstimatedFutureAmortizationExpenseDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/GoodwillAndOtherIntangibleAssetsNetTables" xlink:href="ftnt-20160930.xsd#GoodwillAndOtherIntangibleAssetsNetTables" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/IncomeTaxes" xlink:href="ftnt-20160930.xsd#IncomeTaxes" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/IncomeTaxesDetails" xlink:href="ftnt-20160930.xsd#IncomeTaxesDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/Inventory" xlink:href="ftnt-20160930.xsd#Inventory" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/InventoryDetails" xlink:href="ftnt-20160930.xsd#InventoryDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/InventoryTables" xlink:href="ftnt-20160930.xsd#InventoryTables" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/InvestmentsInPrivatelyHeldCompanies" xlink:href="ftnt-20160930.xsd#InvestmentsInPrivatelyHeldCompanies" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/InvestmentsInPrivatelyHeldCompaniesDetails" xlink:href="ftnt-20160930.xsd#InvestmentsInPrivatelyHeldCompaniesDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/NetIncomePerShare" xlink:href="ftnt-20160930.xsd#NetIncomePerShare" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/NetIncomePerShareAntiDilutiveSecuritiesDetails" xlink:href="ftnt-20160930.xsd#NetIncomePerShareAntiDilutiveSecuritiesDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/NetIncomePerShareCalculationOfBasicAndDilutedDetails" xlink:href="ftnt-20160930.xsd#NetIncomePerShareCalculationOfBasicAndDilutedDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/NetIncomePerShareTables" xlink:href="ftnt-20160930.xsd#NetIncomePerShareTables" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/PropertyAndEquipmentnet" xlink:href="ftnt-20160930.xsd#PropertyAndEquipmentnet" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/PropertyAndEquipmentnetDetails" xlink:href="ftnt-20160930.xsd#PropertyAndEquipmentnetDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/PropertyAndEquipmentnetTables" xlink:href="ftnt-20160930.xsd#PropertyAndEquipmentnetTables" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/RelatedPartyTransactions" xlink:href="ftnt-20160930.xsd#RelatedPartyTransactions" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/RelatedPartyTransactionsDetails" xlink:href="ftnt-20160930.xsd#RelatedPartyTransactionsDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/RestructuringCharges" xlink:href="ftnt-20160930.xsd#RestructuringCharges" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/RestructuringChargesAdditionalInformationDetails" xlink:href="ftnt-20160930.xsd#RestructuringChargesAdditionalInformationDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/RestructuringChargesRestructuringActivityDetails" xlink:href="ftnt-20160930.xsd#RestructuringChargesRestructuringActivityDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/RestructuringChargesTables" xlink:href="ftnt-20160930.xsd#RestructuringChargesTables" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/SegmentAndSignificantCustomerInformation" xlink:href="ftnt-20160930.xsd#SegmentAndSignificantCustomerInformation" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/SegmentAndSignificantCustomerInformationDetails" xlink:href="ftnt-20160930.xsd#SegmentAndSignificantCustomerInformationDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/SegmentAndSignificantCustomerInformationSignificantCustomersDetails" xlink:href="ftnt-20160930.xsd#SegmentAndSignificantCustomerInformationSignificantCustomersDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/SegmentAndSignificantCustomerInformationTables" xlink:href="ftnt-20160930.xsd#SegmentAndSignificantCustomerInformationTables" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/StockholdersEquity" xlink:href="ftnt-20160930.xsd#StockholdersEquity" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/StockholdersEquityAllocationOfStockBasedCompensationExpenseDetails" xlink:href="ftnt-20160930.xsd#StockholdersEquityAllocationOfStockBasedCompensationExpenseDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/StockholdersEquityEmployeeStockOptionsActivityDetails" xlink:href="ftnt-20160930.xsd#StockholdersEquityEmployeeStockOptionsActivityDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/StockholdersEquityEsppInformationDetails" xlink:href="ftnt-20160930.xsd#StockholdersEquityEsppInformationDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/StockholdersEquityRestrictedStockUnitsActivityDetails" xlink:href="ftnt-20160930.xsd#StockholdersEquityRestrictedStockUnitsActivityDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/StockholdersEquityShareRepurchaseProgramDetails" xlink:href="ftnt-20160930.xsd#StockholdersEquityShareRepurchaseProgramDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/StockholdersEquityStockBasedCompensationPlanDetails" xlink:href="ftnt-20160930.xsd#StockholdersEquityStockBasedCompensationPlanDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/StockholdersEquityTables" xlink:href="ftnt-20160930.xsd#StockholdersEquityTables" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/SubsequentEvent" xlink:href="ftnt-20160930.xsd#SubsequentEvent" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/SubsequentEventDetails" xlink:href="ftnt-20160930.xsd#SubsequentEventDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/SummaryOfSignificantAccountingPolicies" xlink:href="ftnt-20160930.xsd#SummaryOfSignificantAccountingPolicies" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/SummaryOfSignificantAccountingPoliciesImpactOfAdoptionOfNewAccountingStandardDetails" xlink:href="ftnt-20160930.xsd#SummaryOfSignificantAccountingPoliciesImpactOfAdoptionOfNewAccountingStandardDetails" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/SummaryOfSignificantAccountingPoliciesPolicies" xlink:href="ftnt-20160930.xsd#SummaryOfSignificantAccountingPoliciesPolicies" xlink:type="simple" />
  <roleRef roleURI="http://www.fortinet.com/role/SummaryOfSignificantAccountingPoliciesTables" xlink:href="ftnt-20160930.xsd#SummaryOfSignificantAccountingPoliciesTables" xlink:type="simple" />
  <arcroleRef arcroleURI="http://xbrl.org/int/dim/arcrole/all" xlink:href="http://www.xbrl.org/2005/xbrldt-2005.xsd#all" xlink:type="simple" />
  <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" />
  <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" />
  <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" />
  <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" />
  <definitionLink xlink:role="http://www.fortinet.com/role/AccumulatedOtherComprehensiveIncomeLoss" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/AccumulatedOtherComprehensiveIncomeLossChangesInAccumulatedBalancesOfOtherComprehensiveIncomedetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_566C1DAAF1AB1F78565845331B27CC16" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossTable" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_DC60CED9142B5B4704EC45331B272777" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_566C1DAAF1AB1F78565845331B27CC16" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_DC60CED9142B5B4704EC45331B272777" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_953C6275F80596A9A8A545331B274E5D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossTable_DC60CED9142B5B4704EC45331B272777" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_953C6275F80596A9A8A545331B274E5D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_E02FC62E2986C7871C2A45331B2715D3_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_953C6275F80596A9A8A545331B274E5D" xlink:to="loc_us-gaap_EquityComponentDomain_E02FC62E2986C7871C2A45331B2715D3_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_E02FC62E2986C7871C2A45331B2715D3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_953C6275F80596A9A8A545331B274E5D" xlink:to="loc_us-gaap_EquityComponentDomain_E02FC62E2986C7871C2A45331B2715D3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember" xlink:label="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_3D6B11579875C3F151FE45331B270565" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_E02FC62E2986C7871C2A45331B2715D3" xlink:to="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_3D6B11579875C3F151FE45331B270565" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_AccumulatedOtherComprehensiveIncomeRollForward" xlink:label="loc_ftnt_AccumulatedOtherComprehensiveIncomeRollForward_DBD044DC996C4FBCAF9445331B276DF7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_566C1DAAF1AB1F78565845331B27CC16" xlink:to="loc_ftnt_AccumulatedOtherComprehensiveIncomeRollForward_DBD044DC996C4FBCAF9445331B276DF7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1" xlink:label="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1_2EA39FE8B1AF8B93CA1F45331B27DA93" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_AccumulatedOtherComprehensiveIncomeRollForward_DBD044DC996C4FBCAF9445331B276DF7" xlink:to="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossBeforeTax1_2EA39FE8B1AF8B93CA1F45331B27DA93" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax_055F175829E543CB756245331B2736A4" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_AccumulatedOtherComprehensiveIncomeRollForward_DBD044DC996C4FBCAF9445331B276DF7" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsBeforeTax_055F175829E543CB756245331B2736A4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax" xlink:label="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax_BBC481E5F4897F754EB745331B273572" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_AccumulatedOtherComprehensiveIncomeRollForward_DBD044DC996C4FBCAF9445331B276DF7" xlink:to="loc_us-gaap_ReclassificationFromAccumulatedOtherComprehensiveIncomeCurrentPeriodBeforeTax_BBC481E5F4897F754EB745331B273572" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeTax_143352161B2BB892114945331B2711EE" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_AccumulatedOtherComprehensiveIncomeRollForward_DBD044DC996C4FBCAF9445331B276DF7" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeTax_143352161B2BB892114945331B2711EE" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_AccumulatedOtherComprehensiveIncomeTaxRollForward" xlink:label="loc_ftnt_AccumulatedOtherComprehensiveIncomeTaxRollForward_3B9168175900F52FE5DF45331B28B9D2" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AccumulatedOtherComprehensiveIncomeLossLineItems_566C1DAAF1AB1F78565845331B27CC16" xlink:to="loc_ftnt_AccumulatedOtherComprehensiveIncomeTaxRollForward_3B9168175900F52FE5DF45331B28B9D2" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_AccumulatedOtherComprehensiveIncomeLossTax" xlink:label="loc_ftnt_AccumulatedOtherComprehensiveIncomeLossTax_7D5920892FE3A6F2028945331B28541E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_AccumulatedOtherComprehensiveIncomeTaxRollForward_3B9168175900F52FE5DF45331B28B9D2" xlink:to="loc_ftnt_AccumulatedOtherComprehensiveIncomeLossTax_7D5920892FE3A6F2028945331B28541E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsTax_74E828335BEBC16C0ED345331B28D0A4" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_AccumulatedOtherComprehensiveIncomeTaxRollForward_3B9168175900F52FE5DF45331B28B9D2" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossBeforeReclassificationsTax_74E828335BEBC16C0ED345331B28D0A4" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_ReclassificationfromAccumulatedOtherComprehensiveIncomeCurrentPeriodTax" xlink:label="loc_ftnt_ReclassificationfromAccumulatedOtherComprehensiveIncomeCurrentPeriodTax_651E0978E84335C4EB4445331B28FA14" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_AccumulatedOtherComprehensiveIncomeTaxRollForward_3B9168175900F52FE5DF45331B28B9D2" xlink:to="loc_ftnt_ReclassificationfromAccumulatedOtherComprehensiveIncomeCurrentPeriodTax_651E0978E84335C4EB4445331B28FA14" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherComprehensiveIncomeLossTax" xlink:label="loc_us-gaap_OtherComprehensiveIncomeLossTax_DFE7CE282976B2E1B8BE45331B287B45" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_AccumulatedOtherComprehensiveIncomeTaxRollForward_3B9168175900F52FE5DF45331B28B9D2" xlink:to="loc_us-gaap_OtherComprehensiveIncomeLossTax_DFE7CE282976B2E1B8BE45331B287B45" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/AccumulatedOtherComprehensiveIncomeLossReclassificationOutOfAccumulatedOtherComprehensiveIncomeDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems" xlink:label="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_7621A2AD475DE6234EF080A2AD4DA48B" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_EFA06E7B6C12E92F23A080A2AD4C2E6A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_7621A2AD475DE6234EF080A2AD4DA48B" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_EFA06E7B6C12E92F23A080A2AD4C2E6A" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_66B9F93F20D5F822969380A2AD4DF203" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_EFA06E7B6C12E92F23A080A2AD4C2E6A" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_66B9F93F20D5F822969380A2AD4DF203" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_E64128579684A4298D6080A2AD4DBF8E_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_66B9F93F20D5F822969380A2AD4DF203" xlink:to="loc_us-gaap_EquityComponentDomain_E64128579684A4298D6080A2AD4DBF8E_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_E64128579684A4298D6080A2AD4DBF8E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_66B9F93F20D5F822969380A2AD4DF203" xlink:to="loc_us-gaap_EquityComponentDomain_E64128579684A4298D6080A2AD4DBF8E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember" xlink:label="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_EE65034D40DDEBAFDBA780A2AD4D81D0" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_E64128579684A4298D6080A2AD4DBF8E" xlink:to="loc_us-gaap_AccumulatedNetUnrealizedInvestmentGainLossMember_EE65034D40DDEBAFDBA780A2AD4D81D0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_425E81CF11C3E784CA9180A2AD4DC80D" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeTable_EFA06E7B6C12E92F23A080A2AD4C2E6A" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_425E81CF11C3E784CA9180A2AD4DC80D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_78A286790ABEC625F1B380A2AD4DFF97_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_425E81CF11C3E784CA9180A2AD4DC80D" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_78A286790ABEC625F1B380A2AD4DFF97_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_78A286790ABEC625F1B380A2AD4DFF97" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeAxis_425E81CF11C3E784CA9180A2AD4DC80D" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_78A286790ABEC625F1B380A2AD4DFF97" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember" xlink:label="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_3FE1A9AFBA63C934BCEA80A2AD4DC217" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeDomain_78A286790ABEC625F1B380A2AD4DFF97" xlink:to="loc_us-gaap_ReclassificationOutOfAccumulatedOtherComprehensiveIncomeMember_3FE1A9AFBA63C934BCEA80A2AD4DC217" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherNonoperatingIncomeExpense" xlink:label="loc_us-gaap_OtherNonoperatingIncomeExpense_255567715C6CFC13BC0380A2AD4DE421" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_7621A2AD475DE6234EF080A2AD4DA48B" xlink:to="loc_us-gaap_OtherNonoperatingIncomeExpense_255567715C6CFC13BC0380A2AD4DE421" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_379F60ADD3ED10116AFD80A2AD4D100C" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_7621A2AD475DE6234EF080A2AD4DA48B" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_379F60ADD3ED10116AFD80A2AD4D100C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_0295B2BAC2BD7685505380A2AD4D75D7" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ReclassificationAdjustmentOutOfAccumulatedOtherComprehensiveIncomeLineItems_7621A2AD475DE6234EF080A2AD4DA48B" xlink:to="loc_us-gaap_NetIncomeLoss_0295B2BAC2BD7685505380A2AD4D75D7" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/AccumulatedOtherComprehensiveIncomeLossTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/BusinessCombinations" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/BusinessCombinationsAdditionalInformationDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_522BDA026609E01135E745331B830904" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3C5481012F4A64AB148145331B82BAC3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_522BDA026609E01135E745331B830904" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3C5481012F4A64AB148145331B82BAC3" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_D508E5042D78DCCC430C45331B825D46" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3C5481012F4A64AB148145331B82BAC3" xlink:to="loc_us-gaap_AwardTypeAxis_D508E5042D78DCCC430C45331B825D46" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4EDA427685E4CD95057445331B823E31_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_D508E5042D78DCCC430C45331B825D46" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4EDA427685E4CD95057445331B823E31_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4EDA427685E4CD95057445331B823E31" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_D508E5042D78DCCC430C45331B825D46" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4EDA427685E4CD95057445331B823E31" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_EE8D18D37C9563A7027845331B820DDE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4EDA427685E4CD95057445331B823E31" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_EE8D18D37C9563A7027845331B820DDE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_C01FD1EB9686E7D01D8E45331B821B81" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3C5481012F4A64AB148145331B82BAC3" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_C01FD1EB9686E7D01D8E45331B821B81" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_3712DB9B15B78D7A79DC45331B82D909_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_C01FD1EB9686E7D01D8E45331B821B81" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_3712DB9B15B78D7A79DC45331B82D909_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_3712DB9B15B78D7A79DC45331B82D909" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_C01FD1EB9686E7D01D8E45331B821B81" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_3712DB9B15B78D7A79DC45331B82D909" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_GeneralAndAdministrativeExpenseMember" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpenseMember_6B83BBCEC9A5EB53CD0645331B82C925" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_3712DB9B15B78D7A79DC45331B82D909" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpenseMember_6B83BBCEC9A5EB53CD0645331B82C925" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_BC7793108ABD89C034FC45331B829BAE" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_3C5481012F4A64AB148145331B82BAC3" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_BC7793108ABD89C034FC45331B829BAE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_CCC5D187BB01098BF36845331B82DE06_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_BC7793108ABD89C034FC45331B829BAE" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_CCC5D187BB01098BF36845331B82DE06_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_CCC5D187BB01098BF36845331B82DE06" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_BC7793108ABD89C034FC45331B829BAE" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_CCC5D187BB01098BF36845331B82DE06" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_AccelOpsInc.Member" xlink:label="loc_ftnt_AccelOpsInc.Member_2E65F6E03F64BEEDD4C645331B831683" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_CCC5D187BB01098BF36845331B82DE06" xlink:to="loc_ftnt_AccelOpsInc.Member_2E65F6E03F64BEEDD4C645331B831683" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_MeruNetworksInc.Member" xlink:label="loc_ftnt_MeruNetworksInc.Member_9656852FB69D3941288645331B83C6DE" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_CCC5D187BB01098BF36845331B82DE06" xlink:to="loc_ftnt_MeruNetworksInc.Member_9656852FB69D3941288645331B83C6DE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationConsiderationTransferred1" xlink:label="loc_us-gaap_BusinessCombinationConsiderationTransferred1_D6E2C3F5B7F25167FEA945331B83BDE2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_522BDA026609E01135E745331B830904" xlink:to="loc_us-gaap_BusinessCombinationConsiderationTransferred1_D6E2C3F5B7F25167FEA945331B83BDE2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired" xlink:label="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_44CD013354DFAA5108B945331B83CF49" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_522BDA026609E01135E745331B830904" xlink:to="loc_us-gaap_PaymentsToAcquireBusinessesNetOfCashAcquired_44CD013354DFAA5108B945331B83CF49" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet_39176C0DA139E9DD500945331B83A867" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_522BDA026609E01135E745331B830904" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet_39176C0DA139E9DD500945331B83A867" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_BusinessCombinationConsiderationTransferredEstimatedFairValueofSharesWithheldforTaxes" xlink:label="loc_ftnt_BusinessCombinationConsiderationTransferredEstimatedFairValueofSharesWithheldforTaxes_71326DF2AAEDF17364B045331B83A541" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_522BDA026609E01135E745331B830904" xlink:to="loc_ftnt_BusinessCombinationConsiderationTransferredEstimatedFairValueofSharesWithheldforTaxes_71326DF2AAEDF17364B045331B83A541" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued" xlink:label="loc_us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued_45500746B1CC591C13A245331B83753D" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_522BDA026609E01135E745331B830904" xlink:to="loc_us-gaap_BusinessAcquisitionEquityInterestsIssuedOrIssuableNumberOfSharesIssued_45500746B1CC591C13A245331B83753D" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_BusinessCombinationConsiderationTransferredFairValueEarnedEquityAwardsAssumed" xlink:label="loc_ftnt_BusinessCombinationConsiderationTransferredFairValueEarnedEquityAwardsAssumed_69B2871951CAFF0168FC45331B83ADEC" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_522BDA026609E01135E745331B830904" xlink:to="loc_ftnt_BusinessCombinationConsiderationTransferredFairValueEarnedEquityAwardsAssumed_69B2871951CAFF0168FC45331B83ADEC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationAcquisitionRelatedCosts" xlink:label="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_F86519CCE26BD568C3FD45331B83B00F" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_522BDA026609E01135E745331B830904" xlink:to="loc_us-gaap_BusinessCombinationAcquisitionRelatedCosts_F86519CCE26BD568C3FD45331B83B00F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_1ADA543F8DA08D649A6B45331B83A009" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_522BDA026609E01135E745331B830904" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_1ADA543F8DA08D649A6B45331B83A009" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_Goodwill" xlink:label="loc_us-gaap_Goodwill_9B0FD2E4A828AFBD032F45331B83DD0A" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_522BDA026609E01135E745331B830904" xlink:to="loc_us-gaap_Goodwill_9B0FD2E4A828AFBD032F45331B83DD0A" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/BusinessCombinationsIntangibleAssetsAcquiredDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_B83F17103B95E5D33F4880A2AD2A8AC0" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_658CC76A5442B689A48080A2AD2A7001" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_B83F17103B95E5D33F4880A2AD2A8AC0" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_658CC76A5442B689A48080A2AD2A7001" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_DB1B1E02A7569526AD4980A2AD2A47D7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_658CC76A5442B689A48080A2AD2A7001" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_DB1B1E02A7569526AD4980A2AD2A47D7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_FC84F23F11BF2289613E80A2AD2A33BC_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_DB1B1E02A7569526AD4980A2AD2A47D7" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_FC84F23F11BF2289613E80A2AD2A33BC_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_FC84F23F11BF2289613E80A2AD2A33BC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_DB1B1E02A7569526AD4980A2AD2A47D7" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_FC84F23F11BF2289613E80A2AD2A33BC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TechnologyBasedIntangibleAssetsMember" xlink:label="loc_us-gaap_TechnologyBasedIntangibleAssetsMember_65B6B49AC5A5BA4A042680A2AD2A2312" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_FC84F23F11BF2289613E80A2AD2A33BC" xlink:to="loc_us-gaap_TechnologyBasedIntangibleAssetsMember_65B6B49AC5A5BA4A042680A2AD2A2312" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CustomerRelationshipsMember" xlink:label="loc_us-gaap_CustomerRelationshipsMember_AE19F777DA9F699B7DE380A2AD2AB586" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_FC84F23F11BF2289613E80A2AD2A33BC" xlink:to="loc_us-gaap_CustomerRelationshipsMember_AE19F777DA9F699B7DE380A2AD2AB586" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherIntangibleAssetsMember" xlink:label="loc_us-gaap_OtherIntangibleAssetsMember_D78FF8F18D6C7DC6072380A2AD2A8683" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_FC84F23F11BF2289613E80A2AD2A33BC" xlink:to="loc_us-gaap_OtherIntangibleAssetsMember_D78FF8F18D6C7DC6072380A2AD2A8683" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TradeNamesMember" xlink:label="loc_us-gaap_TradeNamesMember_86F69969B7D290A5781480A2AD2A4F23" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_FC84F23F11BF2289613E80A2AD2A33BC" xlink:to="loc_us-gaap_TradeNamesMember_86F69969B7D290A5781480A2AD2A4F23" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_94377AE9E730109E96E580A2AD2A72D9" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_658CC76A5442B689A48080A2AD2A7001" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_94377AE9E730109E96E580A2AD2A72D9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_FD531FC4FB0317D005FD80A2AD2A8232_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_94377AE9E730109E96E580A2AD2A72D9" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_FD531FC4FB0317D005FD80A2AD2A8232_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_FD531FC4FB0317D005FD80A2AD2A8232" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_94377AE9E730109E96E580A2AD2A72D9" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_FD531FC4FB0317D005FD80A2AD2A8232" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_AccelOpsInc.Member" xlink:label="loc_ftnt_AccelOpsInc.Member_739C66524B0C339764D380A2AD2AA5EA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_FD531FC4FB0317D005FD80A2AD2A8232" xlink:to="loc_ftnt_AccelOpsInc.Member_739C66524B0C339764D380A2AD2AA5EA" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_MeruNetworksInc.Member" xlink:label="loc_ftnt_MeruNetworksInc.Member_2FD72D7FD23D3679D39C80A2AD2AB05C" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_FD531FC4FB0317D005FD80A2AD2A8232" xlink:to="loc_ftnt_MeruNetworksInc.Member_2FD72D7FD23D3679D39C80A2AD2AB05C" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_FiniteLivedIntangibleAssetEstimatedRemainingUsefulLife" xlink:label="loc_ftnt_FiniteLivedIntangibleAssetEstimatedRemainingUsefulLife_AE9BA02A76237F93B0CE80A2AD2AC5D2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_B83F17103B95E5D33F4880A2AD2A8AC0" xlink:to="loc_ftnt_FiniteLivedIntangibleAssetEstimatedRemainingUsefulLife_AE9BA02A76237F93B0CE80A2AD2AC5D2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FinitelivedIntangibleAssetsAcquired1" xlink:label="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_D05510A6858E55DAA0C880A2AD2BC75D" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_B83F17103B95E5D33F4880A2AD2A8AC0" xlink:to="loc_us-gaap_FinitelivedIntangibleAssetsAcquired1_D05510A6858E55DAA0C880A2AD2BC75D" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/BusinessCombinationsPurchasePriceAllocationsDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_3F9D9B0A41DA2B4BE0E380A2AD162193" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_EB7F9A7A4D19BC38B8C980A2AD15D9D2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3F9D9B0A41DA2B4BE0E380A2AD162193" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_EB7F9A7A4D19BC38B8C980A2AD15D9D2" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_C500B5A87DC79AE98ED380A2AD159981" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_EB7F9A7A4D19BC38B8C980A2AD15D9D2" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_C500B5A87DC79AE98ED380A2AD159981" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_DC8D5F719DE5CB63F46F80A2AD154857_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_C500B5A87DC79AE98ED380A2AD159981" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_DC8D5F719DE5CB63F46F80A2AD154857_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_DC8D5F719DE5CB63F46F80A2AD154857" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_C500B5A87DC79AE98ED380A2AD159981" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_DC8D5F719DE5CB63F46F80A2AD154857" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_AccelOpsInc.Member" xlink:label="loc_ftnt_AccelOpsInc.Member_D1F20971CD6373F97F4A80A2AD16A013" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_DC8D5F719DE5CB63F46F80A2AD154857" xlink:to="loc_ftnt_AccelOpsInc.Member_D1F20971CD6373F97F4A80A2AD16A013" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_MeruNetworksInc.Member" xlink:label="loc_ftnt_MeruNetworksInc.Member_4457A38D57888BB3C66880A2AD161C3F" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_DC8D5F719DE5CB63F46F80A2AD154857" xlink:to="loc_ftnt_MeruNetworksInc.Member_4457A38D57888BB3C66880A2AD161C3F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_1B883C1CFD00F4A9BCCD80A2AD161645" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3F9D9B0A41DA2B4BE0E380A2AD162193" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCashAndEquivalents_1B883C1CFD00F4A9BCCD80A2AD161645" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_207B05105B1264F1187580A2AD161C4C" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3F9D9B0A41DA2B4BE0E380A2AD162193" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsReceivables_207B05105B1264F1187580A2AD161C4C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInventory" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInventory_1756F4FD0435B16CB01680A2AD1643BA" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3F9D9B0A41DA2B4BE0E380A2AD162193" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedInventory_1756F4FD0435B16CB01680A2AD1643BA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets_46C11EBFAB08DBC1C60180A2AD16DFE6" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3F9D9B0A41DA2B4BE0E380A2AD162193" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentAssetsPrepaidExpenseAndOtherAssets_46C11EBFAB08DBC1C60180A2AD16DFE6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_08A725729B0DB94247F880A2AD1632E5" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3F9D9B0A41DA2B4BE0E380A2AD162193" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedPropertyPlantAndEquipment_08A725729B0DB94247F880A2AD1632E5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxAssetsNoncurrent" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxAssetsNoncurrent_7169D04AF14FE981DDA780A2AD163A19" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3F9D9B0A41DA2B4BE0E380A2AD162193" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedDeferredTaxAssetsNoncurrent_7169D04AF14FE981DDA780A2AD163A19" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_BCB4741EE781A54C2BE680A2AD160155" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3F9D9B0A41DA2B4BE0E380A2AD162193" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIntangibles_BCB4741EE781A54C2BE680A2AD160155" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets_D6B5C4F509DEB8B5EA4280A2AD164DAE" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3F9D9B0A41DA2B4BE0E380A2AD162193" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedIndefiniteLivedIntangibleAssets_D6B5C4F509DEB8B5EA4280A2AD164DAE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_Goodwill" xlink:label="loc_us-gaap_Goodwill_13A6F616703A0842689080A2AD177645" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3F9D9B0A41DA2B4BE0E380A2AD162193" xlink:to="loc_us-gaap_Goodwill_13A6F616703A0842689080A2AD177645" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets_0CFF5C929CC8549FE5DD80A2AD174B6F" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3F9D9B0A41DA2B4BE0E380A2AD162193" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedAssets_0CFF5C929CC8549FE5DD80A2AD174B6F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesDeferredRevenue" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesDeferredRevenue_24C94B0E407A0FD2F0DF80A2AD1778A0" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3F9D9B0A41DA2B4BE0E380A2AD162193" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesDeferredRevenue_24C94B0E407A0FD2F0DF80A2AD1778A0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_F09234B94BCEF482C87680A2AD17E1B4" xlink:type="locator" />
    <definitionArc order="12" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3F9D9B0A41DA2B4BE0E380A2AD162193" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesAccountsPayable_F09234B94BCEF482C87680A2AD17E1B4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_1233D1F5E54185E2E19D80A2AD17FF89" xlink:type="locator" />
    <definitionArc order="13" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3F9D9B0A41DA2B4BE0E380A2AD162193" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedCurrentLiabilitiesOther_1233D1F5E54185E2E19D80A2AD17FF89" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_3475BF5A923BF0FF827B80A2AD179FF8" xlink:type="locator" />
    <definitionArc order="14" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3F9D9B0A41DA2B4BE0E380A2AD162193" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedLiabilities_3475BF5A923BF0FF827B80A2AD179FF8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet" xlink:label="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet_982F9765CA84274A6AB180A2AD17A0B6" xlink:type="locator" />
    <definitionArc order="15" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_3F9D9B0A41DA2B4BE0E380A2AD162193" xlink:to="loc_us-gaap_BusinessCombinationRecognizedIdentifiableAssetsAcquiredAndLiabilitiesAssumedNet_982F9765CA84274A6AB180A2AD17A0B6" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/BusinessCombinationsTables" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionLineItems" xlink:label="loc_us-gaap_BusinessAcquisitionLineItems_50E4398A81D2F5B648AE80A2ABE34957" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable" xlink:label="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_049D89160A519E9D550C80A2ABE3767E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_50E4398A81D2F5B648AE80A2ABE34957" xlink:to="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_049D89160A519E9D550C80A2ABE3767E" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_FECAF452768A6843D7B180A2ABE3034C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfBusinessAcquisitionsByAcquisitionTable_049D89160A519E9D550C80A2ABE3767E" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_FECAF452768A6843D7B180A2ABE3034C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_05E34F4960FFB75D106E80A2ABE31A0E_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_FECAF452768A6843D7B180A2ABE3034C" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_05E34F4960FFB75D106E80A2ABE31A0E_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_05E34F4960FFB75D106E80A2ABE31A0E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_FECAF452768A6843D7B180A2ABE3034C" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_05E34F4960FFB75D106E80A2ABE31A0E" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_AccelOpsInc.Member" xlink:label="loc_ftnt_AccelOpsInc.Member_E81A913F14389ABAF05A80A2ABE3F4F6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_05E34F4960FFB75D106E80A2ABE31A0E" xlink:to="loc_ftnt_AccelOpsInc.Member_E81A913F14389ABAF05A80A2ABE3F4F6" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_MeruNetworksInc.Member" xlink:label="loc_ftnt_MeruNetworksInc.Member_1F2D1F83B159B6FC4EED80A2ABE33E06" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_05E34F4960FFB75D106E80A2ABE31A0E" xlink:to="loc_ftnt_MeruNetworksInc.Member_1F2D1F83B159B6FC4EED80A2ABE33E06" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfRecognizedIdentifiedAssetsAcquiredAndLiabilitiesAssumedTableTextBlock" xlink:label="loc_us-gaap_ScheduleOfRecognizedIdentifiedAssetsAcquiredAndLiabilitiesAssumedTableTextBlock_25B6AF51BCAF9974C07C80A2ABE3A928" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_50E4398A81D2F5B648AE80A2ABE34957" xlink:to="loc_us-gaap_ScheduleOfRecognizedIdentifiedAssetsAcquiredAndLiabilitiesAssumedTableTextBlock_25B6AF51BCAF9974C07C80A2ABE3A928" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsAcquiredAsPartOfBusinessCombinationTextBlock" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsAcquiredAsPartOfBusinessCombinationTextBlock_2E84DA81E100B82B263580A2ABE4481D" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionLineItems_50E4398A81D2F5B648AE80A2ABE34957" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsAcquiredAsPartOfBusinessCombinationTextBlock_2E84DA81E100B82B263580A2ABE4481D" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/CommitmentsAndContingencies" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/CommitmentsAndContingenciesDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/CommitmentsAndContingenciesMinimumOperatingLeasePaymentsDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/CommitmentsAndContingenciesTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/CommitmentsAndContingenciesWarrantiesDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/CondensedConsolidatedBalanceSheetsUnaudited" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/CondensedConsolidatedBalanceSheetsUnauditedParenthetical" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/CondensedConsolidatedStatementsOfCashFlowsUnaudited" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/CondensedConsolidatedStatementsOfComprehensiveIncomeUnaudited" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/CondensedConsolidatedStatementsOfOperationsUnaudited" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/DefinedContributionPlans" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/DefinedContributionPlansDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/DocumentAndEntityInformation" xlink:type="extended">
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentInformationLineItems" xlink:label="loc_dei_DocumentInformationLineItems_6F2EB12FF4163C1C0F6A80A2AC2F64BD" xlink:type="locator" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentInformationTable" xlink:label="loc_dei_DocumentInformationTable_3A3877A2FCA8CE91CCF280A2AC2F9953" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_dei_DocumentInformationLineItems_6F2EB12FF4163C1C0F6A80A2AC2F64BD" xlink:to="loc_dei_DocumentInformationTable_3A3877A2FCA8CE91CCF280A2AC2F9953" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentInformationDocumentAxis" xlink:label="loc_dei_DocumentInformationDocumentAxis_6E245C061B6263AAF2FA80A2AC2FF799" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_dei_DocumentInformationTable_3A3877A2FCA8CE91CCF280A2AC2F9953" xlink:to="loc_dei_DocumentInformationDocumentAxis_6E245C061B6263AAF2FA80A2AC2FF799" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentDomain" xlink:label="loc_dei_DocumentDomain_9737C884B547DBE4ADCE80A2AC2FBBA0_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_DocumentInformationDocumentAxis_6E245C061B6263AAF2FA80A2AC2FF799" xlink:to="loc_dei_DocumentDomain_9737C884B547DBE4ADCE80A2AC2FBBA0_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentDomain" xlink:label="loc_dei_DocumentDomain_9737C884B547DBE4ADCE80A2AC2FBBA0" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_DocumentInformationDocumentAxis_6E245C061B6263AAF2FA80A2AC2FF799" xlink:to="loc_dei_DocumentDomain_9737C884B547DBE4ADCE80A2AC2FBBA0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityRegistrantName" xlink:label="loc_dei_EntityRegistrantName_536306721EBAEC4CFF5F80A2AC2F32A1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_6F2EB12FF4163C1C0F6A80A2AC2F64BD" xlink:to="loc_dei_EntityRegistrantName_536306721EBAEC4CFF5F80A2AC2F32A1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityCentralIndexKey" xlink:label="loc_dei_EntityCentralIndexKey_E343C070D39734E54D0B80A2AC2F3488" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_6F2EB12FF4163C1C0F6A80A2AC2F64BD" xlink:to="loc_dei_EntityCentralIndexKey_E343C070D39734E54D0B80A2AC2F3488" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentType" xlink:label="loc_dei_DocumentType_5B563DC660BDB12B707880A2AC2F3906" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_6F2EB12FF4163C1C0F6A80A2AC2F64BD" xlink:to="loc_dei_DocumentType_5B563DC660BDB12B707880A2AC2F3906" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentPeriodEndDate" xlink:label="loc_dei_DocumentPeriodEndDate_E54E60838C408EAB2A9780A2AC2F38D2" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_6F2EB12FF4163C1C0F6A80A2AC2F64BD" xlink:to="loc_dei_DocumentPeriodEndDate_E54E60838C408EAB2A9780A2AC2F38D2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_AmendmentFlag" xlink:label="loc_dei_AmendmentFlag_6E46A6D08890BAEC05D180A2AC2F1066" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_6F2EB12FF4163C1C0F6A80A2AC2F64BD" xlink:to="loc_dei_AmendmentFlag_6E46A6D08890BAEC05D180A2AC2F1066" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentFiscalYearFocus" xlink:label="loc_dei_DocumentFiscalYearFocus_27DC4BE45664681A3EDE80A2AC2FE03F" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_6F2EB12FF4163C1C0F6A80A2AC2F64BD" xlink:to="loc_dei_DocumentFiscalYearFocus_27DC4BE45664681A3EDE80A2AC2FE03F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_DocumentFiscalPeriodFocus" xlink:label="loc_dei_DocumentFiscalPeriodFocus_3AAF1865A9EF9266235780A2AC2FB0B6" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_6F2EB12FF4163C1C0F6A80A2AC2F64BD" xlink:to="loc_dei_DocumentFiscalPeriodFocus_3AAF1865A9EF9266235780A2AC2FB0B6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_CurrentFiscalYearEndDate" xlink:label="loc_dei_CurrentFiscalYearEndDate_14F08CA39188D1C3018680A2AC305148" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_6F2EB12FF4163C1C0F6A80A2AC2F64BD" xlink:to="loc_dei_CurrentFiscalYearEndDate_14F08CA39188D1C3018680A2AC305148" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityCurrentReportingStatus" xlink:label="loc_dei_EntityCurrentReportingStatus_143D64D5F84C37E5BFD980A2AC30C8E9" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_6F2EB12FF4163C1C0F6A80A2AC2F64BD" xlink:to="loc_dei_EntityCurrentReportingStatus_143D64D5F84C37E5BFD980A2AC30C8E9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityFilerCategory" xlink:label="loc_dei_EntityFilerCategory_9A38EE59C06FCFE15DF380A2AC30377F" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_6F2EB12FF4163C1C0F6A80A2AC2F64BD" xlink:to="loc_dei_EntityFilerCategory_9A38EE59C06FCFE15DF380A2AC30377F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityCommonStockSharesOutstanding" xlink:label="loc_dei_EntityCommonStockSharesOutstanding_420EAACE720F10797C4A80A2AC30D35D" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_DocumentInformationLineItems_6F2EB12FF4163C1C0F6A80A2AC2F64BD" xlink:to="loc_dei_EntityCommonStockSharesOutstanding_420EAACE720F10797C4A80A2AC30D35D" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/FinancialInstrumentsAndFairValue" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/FinancialInstrumentsAndFairValueFairValueMeasurementsDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_CD74CB19973D5BA0B73F80A2AD35DB49" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable" xlink:label="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_312AF96DBD583C68B46180A2AD33D8E3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_CD74CB19973D5BA0B73F80A2AD35DB49" xlink:to="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_312AF96DBD583C68B46180A2AD33D8E3" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueByMeasurementBasisAxis" xlink:label="loc_us-gaap_FairValueByMeasurementBasisAxis_77BC30A0741322E2F78A80A2AD33648A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_312AF96DBD583C68B46180A2AD33D8E3" xlink:to="loc_us-gaap_FairValueByMeasurementBasisAxis_77BC30A0741322E2F78A80A2AD33648A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_8F2F81F817B871B1DC3280A2AD34B274_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_77BC30A0741322E2F78A80A2AD33648A" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_8F2F81F817B871B1DC3280A2AD34B274_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueDisclosureItemAmountsDomain" xlink:label="loc_us-gaap_FairValueDisclosureItemAmountsDomain_09D9AEC1B86617CAEB5380A2AD3337C9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementBasisAxis_77BC30A0741322E2F78A80A2AD33648A" xlink:to="loc_us-gaap_FairValueDisclosureItemAmountsDomain_09D9AEC1B86617CAEB5380A2AD3337C9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PortionAtFairValueFairValueDisclosureMember" xlink:label="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_8F2F81F817B871B1DC3280A2AD34B274" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueDisclosureItemAmountsDomain_09D9AEC1B86617CAEB5380A2AD3337C9" xlink:to="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_8F2F81F817B871B1DC3280A2AD34B274" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EstimateOfFairValueFairValueDisclosureMember" xlink:label="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_87F60BA782B41F4E455480A2AD341B34" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_8F2F81F817B871B1DC3280A2AD34B274" xlink:to="loc_us-gaap_EstimateOfFairValueFairValueDisclosureMember_87F60BA782B41F4E455480A2AD341B34" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CarryingReportedAmountFairValueDisclosureMember" xlink:label="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_4B8B4687856A73E39BB980A2AD3477A7" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PortionAtFairValueFairValueDisclosureMember_8F2F81F817B871B1DC3280A2AD34B274" xlink:to="loc_us-gaap_CarryingReportedAmountFairValueDisclosureMember_4B8B4687856A73E39BB980A2AD3477A7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis" xlink:label="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_90DF161AE3FFA5F8900080A2AD347B0A" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_312AF96DBD583C68B46180A2AD33D8E3" xlink:to="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_90DF161AE3FFA5F8900080A2AD347B0A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" xlink:label="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_C0A0EF79A5917CE9D46980A2AD34CB93_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_90DF161AE3FFA5F8900080A2AD347B0A" xlink:to="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_C0A0EF79A5917CE9D46980A2AD34CB93_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" xlink:label="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_C0A0EF79A5917CE9D46980A2AD34CB93" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_90DF161AE3FFA5F8900080A2AD347B0A" xlink:to="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_C0A0EF79A5917CE9D46980A2AD34CB93" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CorporateDebtSecuritiesMember" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_92930D5AE02E321979FA80A2AD3472E4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_C0A0EF79A5917CE9D46980A2AD34CB93" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_92930D5AE02E321979FA80A2AD3472E4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember" xlink:label="loc_us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_2A56A4BADA7FA65F4D5180A2AD3412C4" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_C0A0EF79A5917CE9D46980A2AD34CB93" xlink:to="loc_us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_2A56A4BADA7FA65F4D5180A2AD3412C4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:label="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_8D0443214B86A53C0B9680A2AD34E2C0" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_C0A0EF79A5917CE9D46980A2AD34CB93" xlink:to="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_8D0443214B86A53C0B9680A2AD34E2C0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CertificatesOfDepositMember" xlink:label="loc_us-gaap_CertificatesOfDepositMember_43A35DB3CA4EA227B72380A2AD349866" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_C0A0EF79A5917CE9D46980A2AD34CB93" xlink:to="loc_us-gaap_CertificatesOfDepositMember_43A35DB3CA4EA227B72380A2AD349866" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_MoneyMarketFundsExcludingCommercialPaperMember" xlink:label="loc_ftnt_MoneyMarketFundsExcludingCommercialPaperMember_5C8C6A38498BB3B4CB0D80A2AD347674" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_C0A0EF79A5917CE9D46980A2AD34CB93" xlink:to="loc_ftnt_MoneyMarketFundsExcludingCommercialPaperMember_5C8C6A38498BB3B4CB0D80A2AD347674" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_USTreasuryAndGovernmentMember" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_7C5E555751994189AC4A80A2AD3488ED" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_C0A0EF79A5917CE9D46980A2AD34CB93" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_7C5E555751994189AC4A80A2AD3488ED" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueByMeasurementFrequencyAxis" xlink:label="loc_us-gaap_FairValueByMeasurementFrequencyAxis_84C63F9CFC2EFCC741A280A2AD345EC2" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_312AF96DBD583C68B46180A2AD33D8E3" xlink:to="loc_us-gaap_FairValueByMeasurementFrequencyAxis_84C63F9CFC2EFCC741A280A2AD345EC2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_F1514AB5207C351F000A80A2AD34CBD7_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_84C63F9CFC2EFCC741A280A2AD345EC2" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_F1514AB5207C351F000A80A2AD34CBD7_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementFrequencyDomain" xlink:label="loc_us-gaap_FairValueMeasurementFrequencyDomain_F1514AB5207C351F000A80A2AD34CBD7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByMeasurementFrequencyAxis_84C63F9CFC2EFCC741A280A2AD345EC2" xlink:to="loc_us-gaap_FairValueMeasurementFrequencyDomain_F1514AB5207C351F000A80A2AD34CBD7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementsRecurringMember" xlink:label="loc_us-gaap_FairValueMeasurementsRecurringMember_B730EB1F445A4F0F057080A2AD346A1A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementFrequencyDomain_F1514AB5207C351F000A80A2AD34CBD7" xlink:to="loc_us-gaap_FairValueMeasurementsRecurringMember_B730EB1F445A4F0F057080A2AD346A1A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_A3EF16A1FA0A5EAB6FFE80A2AD35497C" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_312AF96DBD583C68B46180A2AD33D8E3" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_A3EF16A1FA0A5EAB6FFE80A2AD35497C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_C7C6275A5CF8825B7F7A80A2AD3526FC_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_A3EF16A1FA0A5EAB6FFE80A2AD35497C" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_C7C6275A5CF8825B7F7A80A2AD3526FC_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_C7C6275A5CF8825B7F7A80A2AD3526FC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_A3EF16A1FA0A5EAB6FFE80A2AD35497C" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_C7C6275A5CF8825B7F7A80A2AD3526FC" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_CashEquivalentsLineItemMember" xlink:label="loc_ftnt_CashEquivalentsLineItemMember_110184D47F29CD9F8E7C80A2AD352D49" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_C7C6275A5CF8825B7F7A80A2AD3526FC" xlink:to="loc_ftnt_CashEquivalentsLineItemMember_110184D47F29CD9F8E7C80A2AD352D49" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_ShortTermInvestmentsLineItemMember" xlink:label="loc_ftnt_ShortTermInvestmentsLineItemMember_F823782EE31B52F7E8D580A2AD3594F3" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_C7C6275A5CF8825B7F7A80A2AD3526FC" xlink:to="loc_ftnt_ShortTermInvestmentsLineItemMember_F823782EE31B52F7E8D580A2AD3594F3" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_LongTermInvestmentsLineItemMember" xlink:label="loc_ftnt_LongTermInvestmentsLineItemMember_729069F10430D438A68680A2AD358141" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_C7C6275A5CF8825B7F7A80A2AD3526FC" xlink:to="loc_ftnt_LongTermInvestmentsLineItemMember_729069F10430D438A68680A2AD358141" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueByFairValueHierarchyLevelAxis" xlink:label="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_2F5DBB85687EA4DE894880A2AD350A09" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisTable_312AF96DBD583C68B46180A2AD33D8E3" xlink:to="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_2F5DBB85687EA4DE894880A2AD350A09" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_7BFE2324DFE7CA8BF81880A2AD3563D3_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_2F5DBB85687EA4DE894880A2AD350A09" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_7BFE2324DFE7CA8BF81880A2AD3563D3_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueMeasurementsFairValueHierarchyDomain" xlink:label="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_7BFE2324DFE7CA8BF81880A2AD3563D3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FairValueByFairValueHierarchyLevelAxis_2F5DBB85687EA4DE894880A2AD350A09" xlink:to="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_7BFE2324DFE7CA8BF81880A2AD3563D3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueInputsLevel1Member" xlink:label="loc_us-gaap_FairValueInputsLevel1Member_7F90B0632456C201146D80A2AD3554D6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_7BFE2324DFE7CA8BF81880A2AD3563D3" xlink:to="loc_us-gaap_FairValueInputsLevel1Member_7F90B0632456C201146D80A2AD3554D6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueInputsLevel2Member" xlink:label="loc_us-gaap_FairValueInputsLevel2Member_F4D2335C95523042E3BE80A2AD351B5E" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_7BFE2324DFE7CA8BF81880A2AD3563D3" xlink:to="loc_us-gaap_FairValueInputsLevel2Member_F4D2335C95523042E3BE80A2AD351B5E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FairValueInputsLevel3Member" xlink:label="loc_us-gaap_FairValueInputsLevel3Member_8898B610F96A3D5C7B9580A2AD35245E" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueMeasurementsFairValueHierarchyDomain_7BFE2324DFE7CA8BF81880A2AD3563D3" xlink:to="loc_us-gaap_FairValueInputsLevel3Member_8898B610F96A3D5C7B9580A2AD35245E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecurities" xlink:label="loc_us-gaap_AvailableForSaleSecurities_174345442A4134D861D980A2AD35015E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_CD74CB19973D5BA0B73F80A2AD35DB49" xlink:to="loc_us-gaap_AvailableForSaleSecurities_174345442A4134D861D980A2AD35015E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AssetsFairValueDisclosure" xlink:label="loc_us-gaap_AssetsFairValueDisclosure_8039A44382A3D72A9EA980A2AD3568CE" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_CD74CB19973D5BA0B73F80A2AD35DB49" xlink:to="loc_us-gaap_AssetsFairValueDisclosure_8039A44382A3D72A9EA980A2AD3568CE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh" xlink:label="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_837B0C287B68BB5C7D7B80A2AD35D800" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FairValueAssetsAndLiabilitiesMeasuredOnRecurringAndNonrecurringBasisLineItems_CD74CB19973D5BA0B73F80A2AD35DB49" xlink:to="loc_us-gaap_BusinessCombinationContingentConsiderationArrangementsRangeOfOutcomesValueHigh_837B0C287B68BB5C7D7B80A2AD35D800" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/FinancialInstrumentsAndFairValueInvestmentsDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_10C8A83CFAE4D8B8B4DD223FBA2BF33A" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfAvailableForSaleSecuritiesTable" xlink:label="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesTable_590DBFE3B53A7C8118A7223FBA29223F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_10C8A83CFAE4D8B8B4DD223FBA2BF33A" xlink:to="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesTable_590DBFE3B53A7C8118A7223FBA29223F" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis" xlink:label="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_3F98F259F5C9B5E7E40C223FBA2A43A3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesTable_590DBFE3B53A7C8118A7223FBA29223F" xlink:to="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_3F98F259F5C9B5E7E40C223FBA2A43A3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" xlink:label="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_715CB12E1E370C415315223FBA2A0EE1_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_3F98F259F5C9B5E7E40C223FBA2A43A3" xlink:to="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_715CB12E1E370C415315223FBA2A0EE1_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain" xlink:label="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_715CB12E1E370C415315223FBA2A0EE1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesAxis_3F98F259F5C9B5E7E40C223FBA2A43A3" xlink:to="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_715CB12E1E370C415315223FBA2A0EE1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CorporateDebtSecuritiesMember" xlink:label="loc_us-gaap_CorporateDebtSecuritiesMember_5F6222DD1349332B90B0223FBA2A15A9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_715CB12E1E370C415315223FBA2A0EE1" xlink:to="loc_us-gaap_CorporateDebtSecuritiesMember_5F6222DD1349332B90B0223FBA2A15A9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember" xlink:label="loc_us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_4D74FC814EE9FBDD6733223FBA2A30F4" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_715CB12E1E370C415315223FBA2A0EE1" xlink:to="loc_us-gaap_CommercialPaperNotIncludedWithCashAndCashEquivalentsMember_4D74FC814EE9FBDD6733223FBA2A30F4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_USStatesAndPoliticalSubdivisionsMember" xlink:label="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_546CC940C6AE68D4E035223FBA2B3E38" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_715CB12E1E370C415315223FBA2A0EE1" xlink:to="loc_us-gaap_USStatesAndPoliticalSubdivisionsMember_546CC940C6AE68D4E035223FBA2B3E38" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CertificatesOfDepositMember" xlink:label="loc_us-gaap_CertificatesOfDepositMember_02975465F85B2BC3D254223FBA2B96CA" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_715CB12E1E370C415315223FBA2A0EE1" xlink:to="loc_us-gaap_CertificatesOfDepositMember_02975465F85B2BC3D254223FBA2B96CA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_USTreasuryAndGovernmentMember" xlink:label="loc_us-gaap_USTreasuryAndGovernmentMember_FF222CF9CFC951A69AA9223FBA2B60ED" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_MajorTypesOfDebtAndEquitySecuritiesDomain_715CB12E1E370C415315223FBA2A0EE1" xlink:to="loc_us-gaap_USTreasuryAndGovernmentMember_FF222CF9CFC951A69AA9223FBA2B60ED" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_E5C941A8BF0712A7C2A6223FBA2BB3C2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_10C8A83CFAE4D8B8B4DD223FBA2BF33A" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAmortizedCostBasis_E5C941A8BF0712A7C2A6223FBA2BB3C2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_FC4A069DEB69E55257FD223FBA2CC547" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_10C8A83CFAE4D8B8B4DD223FBA2BF33A" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedGainBeforeTax_FC4A069DEB69E55257FD223FBA2CC547" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax" xlink:label="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_C763C9BFED4E785C66B2225F4F5C65AB" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_10C8A83CFAE4D8B8B4DD223FBA2BF33A" xlink:to="loc_us-gaap_AvailableForSaleDebtSecuritiesAccumulatedGrossUnrealizedLossBeforeTax_C763C9BFED4E785C66B2225F4F5C65AB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAbstract" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAbstract_8A6225407FD657501174223FBA2C6BBB" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_10C8A83CFAE4D8B8B4DD223FBA2BF33A" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAbstract_8A6225407FD657501174223FBA2C6BBB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue_0271CD6E2B7664383EEC223FBA2CAEAF" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAbstract_8A6225407FD657501174223FBA2C6BBB" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThanTwelveMonthsFairValue_0271CD6E2B7664383EEC223FBA2CAEAF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_E4A23F622CAE16E44536223FBA2DA060" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAbstract_8A6225407FD657501174223FBA2C6BBB" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionLessThan12MonthsAccumulatedLoss_E4A23F622CAE16E44536223FBA2DA060" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue_D30E357E6EC2D8A62655223FBA2DD533" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAbstract_8A6225407FD657501174223FBA2C6BBB" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionTwelveMonthsOrLongerFairValue_D30E357E6EC2D8A62655223FBA2DD533" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_FDB94E134B00E457C85B223FBA2DD607" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAbstract_8A6225407FD657501174223FBA2C6BBB" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPosition12MonthsOrLongerAccumulatedLoss_FDB94E134B00E457C85B223FBA2DD607" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue_CB1A7B1F4F1B221C7A4F223FBA2D9D8E" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAbstract_8A6225407FD657501174223FBA2C6BBB" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionFairValue_CB1A7B1F4F1B221C7A4F223FBA2D9D8E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss_573951CDF85AD986C509223FBA2DEC78" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAbstract_8A6225407FD657501174223FBA2C6BBB" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesContinuousUnrealizedLossPositionAccumulatedLoss_573951CDF85AD986C509223FBA2DEC78" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_C66E20F45355C6730860223FBA2D7CAF" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScheduleOfAvailableForSaleSecuritiesLineItems_10C8A83CFAE4D8B8B4DD223FBA2BF33A" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_C66E20F45355C6730860223FBA2D7CAF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue_4EE8E655CFB0CE3CCCB6223FBA2D2F41" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_C66E20F45355C6730860223FBA2D7CAF" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesWithinOneYearFairValue_4EE8E655CFB0CE3CCCB6223FBA2D2F41" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_AvailableForSaleSecuritiesDebtMaturitiesAfterOneYearFairValue" xlink:label="loc_ftnt_AvailableForSaleSecuritiesDebtMaturitiesAfterOneYearFairValue_CD23DD7F6213B108773D223FBA2D8602" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_C66E20F45355C6730860223FBA2D7CAF" xlink:to="loc_ftnt_AvailableForSaleSecuritiesDebtMaturitiesAfterOneYearFairValue_CD23DD7F6213B108773D223FBA2D8602" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AvailableForSaleSecuritiesDebtSecurities" xlink:label="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_5F4729848147EBB22713223FBA2D6EF0" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AvailableForSaleSecuritiesDebtMaturitiesFairValueAbstract_C66E20F45355C6730860223FBA2D7CAF" xlink:to="loc_us-gaap_AvailableForSaleSecuritiesDebtSecurities_5F4729848147EBB22713223FBA2D6EF0" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/FinancialInstrumentsAndFairValueTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/ForeignCurrencyDerivatives" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/ForeignCurrencyDerivativesDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeLineItems" xlink:label="loc_us-gaap_DerivativeLineItems_6939A2F0D156705F83A880A2ABD0418C" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeTable" xlink:label="loc_us-gaap_DerivativeTable_870573200EEFCD8DB00780A2ABD074EA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_DerivativeLineItems_6939A2F0D156705F83A880A2ABD0418C" xlink:to="loc_us-gaap_DerivativeTable_870573200EEFCD8DB00780A2ABD074EA" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_2895A6821EC0BD95153B80A2ABD06673" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_870573200EEFCD8DB00780A2ABD074EA" xlink:to="loc_us-gaap_RangeAxis_2895A6821EC0BD95153B80A2ABD06673" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_B427DA6560AC660C197480A2ABD06A63_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_2895A6821EC0BD95153B80A2ABD06673" xlink:to="loc_us-gaap_RangeMember_B427DA6560AC660C197480A2ABD06A63_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_B427DA6560AC660C197480A2ABD06A63" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_2895A6821EC0BD95153B80A2ABD06673" xlink:to="loc_us-gaap_RangeMember_B427DA6560AC660C197480A2ABD06A63" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MinimumMember" xlink:label="loc_us-gaap_MinimumMember_4D20B23CB3ED5F74079C80A2ABD0EC48" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_B427DA6560AC660C197480A2ABD06A63" xlink:to="loc_us-gaap_MinimumMember_4D20B23CB3ED5F74079C80A2ABD0EC48" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MaximumMember" xlink:label="loc_us-gaap_MaximumMember_935A0236C33489B53ED880A2ABD0CE50" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_B427DA6560AC660C197480A2ABD06A63" xlink:to="loc_us-gaap_MaximumMember_935A0236C33489B53ED880A2ABD0CE50" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CurrencyAxis" xlink:label="loc_us-gaap_CurrencyAxis_19C5EE04D5F28A8722FC80A2ABD0BFBB" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_870573200EEFCD8DB00780A2ABD074EA" xlink:to="loc_us-gaap_CurrencyAxis_19C5EE04D5F28A8722FC80A2ABD0BFBB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/currency/2014/currency-2014-01-31.xsd#currency_AllCurrenciesDomain" xlink:label="loc_currency_AllCurrenciesDomain_F1DA4F80DED13D78A62A80A2ABD0A146_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_CurrencyAxis_19C5EE04D5F28A8722FC80A2ABD0BFBB" xlink:to="loc_currency_AllCurrenciesDomain_F1DA4F80DED13D78A62A80A2ABD0A146_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/currency/2014/currency-2014-01-31.xsd#currency_AllCurrenciesDomain" xlink:label="loc_currency_AllCurrenciesDomain_F1DA4F80DED13D78A62A80A2ABD0A146" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_CurrencyAxis_19C5EE04D5F28A8722FC80A2ABD0BFBB" xlink:to="loc_currency_AllCurrenciesDomain_F1DA4F80DED13D78A62A80A2ABD0A146" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/currency/2014/currency-2014-01-31.xsd#currency_CAD" xlink:label="loc_currency_CAD_98318681875DAD044A7F80A2ABD0B1C2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_currency_AllCurrenciesDomain_F1DA4F80DED13D78A62A80A2ABD0A146" xlink:to="loc_currency_CAD_98318681875DAD044A7F80A2ABD0B1C2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PositionAxis" xlink:label="loc_us-gaap_PositionAxis_4527F3DE45E3DD6C4BD480A2ABD0398E" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_DerivativeTable_870573200EEFCD8DB00780A2ABD074EA" xlink:to="loc_us-gaap_PositionAxis_4527F3DE45E3DD6C4BD480A2ABD0398E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PositionDomain" xlink:label="loc_us-gaap_PositionDomain_C5F39FCA92A472DDCC6D80A2ABD004ED_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PositionAxis_4527F3DE45E3DD6C4BD480A2ABD0398E" xlink:to="loc_us-gaap_PositionDomain_C5F39FCA92A472DDCC6D80A2ABD004ED_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PositionDomain" xlink:label="loc_us-gaap_PositionDomain_C5F39FCA92A472DDCC6D80A2ABD004ED" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PositionAxis_4527F3DE45E3DD6C4BD480A2ABD0398E" xlink:to="loc_us-gaap_PositionDomain_C5F39FCA92A472DDCC6D80A2ABD004ED" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShortMember" xlink:label="loc_us-gaap_ShortMember_56C93615759EC8A7623980A2ABD016F8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PositionDomain_C5F39FCA92A472DDCC6D80A2ABD004ED" xlink:to="loc_us-gaap_ShortMember_56C93615759EC8A7623980A2ABD016F8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DerivativeTermOfContract" xlink:label="loc_us-gaap_DerivativeTermOfContract_5C8DB18AC51B5AE3CE1680A2ABD03D21" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_6939A2F0D156705F83A880A2ABD0418C" xlink:to="loc_us-gaap_DerivativeTermOfContract_5C8DB18AC51B5AE3CE1680A2ABD03D21" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/invest/2013/invest-2013-01-31.xsd#invest_DerivativeNotionalAmount" xlink:label="loc_invest_DerivativeNotionalAmount_4D679840EA5600E1194480A2ABD1539F" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_DerivativeLineItems_6939A2F0D156705F83A880A2ABD0418C" xlink:to="loc_invest_DerivativeNotionalAmount_4D679840EA5600E1194480A2ABD1539F" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/ForeignCurrencyDerivativesTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/GoodwillAndOtherIntangibleAssetsNet" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/GoodwillAndOtherIntangibleAssetsNetChangesInCarryingAmountOfGoodwillDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/GoodwillAndOtherIntangibleAssetsNetDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsLineItems" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_383B0C119A2B9F1DD2212C7BEAB27B9F" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable" xlink:label="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_4B9435BB616048EED3132C7BEAB2A315" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_383B0C119A2B9F1DD2212C7BEAB27B9F" xlink:to="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_4B9435BB616048EED3132C7BEAB2A315" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_2E27C82BD54DC29EDE8D2C7BEAB2B99F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_4B9435BB616048EED3132C7BEAB2A315" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_2E27C82BD54DC29EDE8D2C7BEAB2B99F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_B05910BC34870AABAE402C7BEAB21CE3_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_2E27C82BD54DC29EDE8D2C7BEAB2B99F" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_B05910BC34870AABAE402C7BEAB21CE3_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_B05910BC34870AABAE402C7BEAB21CE3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsByMajorClassAxis_2E27C82BD54DC29EDE8D2C7BEAB2B99F" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_B05910BC34870AABAE402C7BEAB21CE3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_InProcessResearchAndDevelopmentMember" xlink:label="loc_us-gaap_InProcessResearchAndDevelopmentMember_3F9D65767C786BE945492C7BEAB2FD37" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsMajorClassNameDomain_B05910BC34870AABAE402C7BEAB21CE3" xlink:to="loc_us-gaap_InProcessResearchAndDevelopmentMember_3F9D65767C786BE945492C7BEAB2FD37" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_0F08B151598A89F5A4142C7BEAB277DE" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfFiniteLivedIntangibleAssetsTable_4B9435BB616048EED3132C7BEAB2A315" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_0F08B151598A89F5A4142C7BEAB277DE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_5376264E3D6581F0ADDD2C7BEAB26CBE_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_0F08B151598A89F5A4142C7BEAB277DE" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_5376264E3D6581F0ADDD2C7BEAB26CBE_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_5376264E3D6581F0ADDD2C7BEAB26CBE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsByMajorClassAxis_0F08B151598A89F5A4142C7BEAB277DE" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_5376264E3D6581F0ADDD2C7BEAB26CBE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CustomerRelationshipsMember" xlink:label="loc_us-gaap_CustomerRelationshipsMember_471F17FFA0907ED2EC132C7BEAB23504" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_5376264E3D6581F0ADDD2C7BEAB26CBE" xlink:to="loc_us-gaap_CustomerRelationshipsMember_471F17FFA0907ED2EC132C7BEAB23504" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DevelopedTechnologyRightsMember" xlink:label="loc_us-gaap_DevelopedTechnologyRightsMember_2DCCAF9C031CCD000B7B2C7BEAB2FB62" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsMajorClassNameDomain_5376264E3D6581F0ADDD2C7BEAB26CBE" xlink:to="loc_us-gaap_DevelopedTechnologyRightsMember_2DCCAF9C031CCD000B7B2C7BEAB2FB62" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_Goodwill" xlink:label="loc_us-gaap_Goodwill_8CA925ACE3832D9A38812C7BEAB2CA1D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_383B0C119A2B9F1DD2212C7BEAB27B9F" xlink:to="loc_us-gaap_Goodwill_8CA925ACE3832D9A38812C7BEAB2CA1D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_GoodwillImpairmentLoss" xlink:label="loc_us-gaap_GoodwillImpairmentLoss_8C62CA7C1ADE3CF087CD2C7BEAB2C6FD" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_383B0C119A2B9F1DD2212C7BEAB27B9F" xlink:to="loc_us-gaap_GoodwillImpairmentLoss_8C62CA7C1ADE3CF087CD2C7BEAB2C6FD" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife" xlink:label="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_50B4CE2046943EA612B92C7BEAB28545" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_383B0C119A2B9F1DD2212C7BEAB27B9F" xlink:to="loc_us-gaap_AcquiredFiniteLivedIntangibleAssetsWeightedAverageUsefulLife_50B4CE2046943EA612B92C7BEAB28545" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsNetAbstract" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_B25F953A1549DA0BDA792C7BEAB38DF4" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_383B0C119A2B9F1DD2212C7BEAB27B9F" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_B25F953A1549DA0BDA792C7BEAB38DF4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsGross" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsGross_6DF3A5EDD7070AD9C9682C7BEAB379D5" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_B25F953A1549DA0BDA792C7BEAB38DF4" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsGross_6DF3A5EDD7070AD9C9682C7BEAB379D5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_0CE1DC9899D2BE40A11B2C7BEAB3F4A2" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_B25F953A1549DA0BDA792C7BEAB38DF4" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsAccumulatedAmortization_0CE1DC9899D2BE40A11B2C7BEAB3F4A2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FiniteLivedIntangibleAssetsNet" xlink:label="loc_us-gaap_FiniteLivedIntangibleAssetsNet_ADFF945D43F9D1C5D9E72C7BEAB3EB74" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsNetAbstract_B25F953A1549DA0BDA792C7BEAB38DF4" xlink:to="loc_us-gaap_FiniteLivedIntangibleAssetsNet_ADFF945D43F9D1C5D9E72C7BEAB3EB74" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwillAbstract" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwillAbstract_E9C9E7203A21A12818822C7BEAB335E1" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_383B0C119A2B9F1DD2212C7BEAB27B9F" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwillAbstract_E9C9E7203A21A12818822C7BEAB335E1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill" xlink:label="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_0D19578D1EE7346A99F92C7BEAB32FDC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwillAbstract_E9C9E7203A21A12818822C7BEAB335E1" xlink:to="loc_us-gaap_IndefiniteLivedIntangibleAssetsExcludingGoodwill_0D19578D1EE7346A99F92C7BEAB32FDC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IntangibleAssetsGrossExcludingGoodwill" xlink:label="loc_us-gaap_IntangibleAssetsGrossExcludingGoodwill_18A8307FC85CA8E40C352C7BEAB39797" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_383B0C119A2B9F1DD2212C7BEAB27B9F" xlink:to="loc_us-gaap_IntangibleAssetsGrossExcludingGoodwill_18A8307FC85CA8E40C352C7BEAB39797" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IntangibleAssetsNetExcludingGoodwill" xlink:label="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_BE0F43A3C89072C99F8E2C7BEAB3BF46" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_383B0C119A2B9F1DD2212C7BEAB27B9F" xlink:to="loc_us-gaap_IntangibleAssetsNetExcludingGoodwill_BE0F43A3C89072C99F8E2C7BEAB3BF46" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AmortizationOfIntangibleAssets" xlink:label="loc_us-gaap_AmortizationOfIntangibleAssets_3E792348594C17CB3C902C7BEAB3F54C" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_FiniteLivedIntangibleAssetsLineItems_383B0C119A2B9F1DD2212C7BEAB27B9F" xlink:to="loc_us-gaap_AmortizationOfIntangibleAssets_3E792348594C17CB3C902C7BEAB3F54C" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/GoodwillAndOtherIntangibleAssetsNetEstimatedFutureAmortizationExpenseDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/GoodwillAndOtherIntangibleAssetsNetTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/IncomeTaxes" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/IncomeTaxesDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems" xlink:label="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_FAC06F476C4D24C007E12C634E84B255" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable" xlink:label="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_BDA0DCF53F9BDED57D5F2C634E84C1D2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_FAC06F476C4D24C007E12C634E84B255" xlink:to="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_BDA0DCF53F9BDED57D5F2C634E84C1D2" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NewAccountingPronouncementEarlyAdoptionAxis" xlink:label="loc_us-gaap_NewAccountingPronouncementEarlyAdoptionAxis_7DB98DAA3540485F24E32C634E849021" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_BDA0DCF53F9BDED57D5F2C634E84C1D2" xlink:to="loc_us-gaap_NewAccountingPronouncementEarlyAdoptionAxis_7DB98DAA3540485F24E32C634E849021" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NewAccountingPrinciplesEarlyAdoptionMember" xlink:label="loc_us-gaap_NewAccountingPrinciplesEarlyAdoptionMember_91B7999AF2BEA707CB512C634E84DE3F_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_NewAccountingPronouncementEarlyAdoptionAxis_7DB98DAA3540485F24E32C634E849021" xlink:to="loc_us-gaap_NewAccountingPrinciplesEarlyAdoptionMember_91B7999AF2BEA707CB512C634E84DE3F_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NewAccountingPrinciplesEarlyAdoptionMember" xlink:label="loc_us-gaap_NewAccountingPrinciplesEarlyAdoptionMember_91B7999AF2BEA707CB512C634E84DE3F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_NewAccountingPronouncementEarlyAdoptionAxis_7DB98DAA3540485F24E32C634E849021" xlink:to="loc_us-gaap_NewAccountingPrinciplesEarlyAdoptionMember_91B7999AF2BEA707CB512C634E84DE3F" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_AccountingStandardsUpdate201609Member" xlink:label="loc_ftnt_AccountingStandardsUpdate201609Member_E0C9C36126B10EDB558B2C634E843E35" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPrinciplesEarlyAdoptionMember_91B7999AF2BEA707CB512C634E84DE3F" xlink:to="loc_ftnt_AccountingStandardsUpdate201609Member_E0C9C36126B10EDB558B2C634E843E35" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EffectiveIncomeTaxRateContinuingOperations" xlink:label="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_FD9E35CDDC0B1EE983F92C634E859A6E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_FAC06F476C4D24C007E12C634E84B255" xlink:to="loc_us-gaap_EffectiveIncomeTaxRateContinuingOperations_FD9E35CDDC0B1EE983F92C634E859A6E" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_EffectiveIncomeTaxRateReconciliationSharebasedCompensationExcessTaxBenefitAmount" xlink:label="loc_ftnt_EffectiveIncomeTaxRateReconciliationSharebasedCompensationExcessTaxBenefitAmount_710E3D776079ACBEE2C92C634E85230C" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_FAC06F476C4D24C007E12C634E84B255" xlink:to="loc_ftnt_EffectiveIncomeTaxRateReconciliationSharebasedCompensationExcessTaxBenefitAmount_710E3D776079ACBEE2C92C634E85230C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_UnrecognizedTaxBenefits" xlink:label="loc_us-gaap_UnrecognizedTaxBenefits_2C335CDB2597912D0C442C634E85E4CE" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_FAC06F476C4D24C007E12C634E84B255" xlink:to="loc_us-gaap_UnrecognizedTaxBenefits_2C335CDB2597912D0C442C634E85E4CE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate_91B4C998E0BCFAC47ED62C634E85DF6C" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_FAC06F476C4D24C007E12C634E84B255" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsThatWouldImpactEffectiveTaxRate_91B4C998E0BCFAC47ED62C634E85DF6C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued" xlink:label="loc_us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued_56FB3FFF04281ADB1AE52C634E854640" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_FAC06F476C4D24C007E12C634E84B255" xlink:to="loc_us-gaap_UnrecognizedTaxBenefitsInterestOnIncomeTaxesAccrued_56FB3FFF04281ADB1AE52C634E854640" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/Inventory" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/InventoryDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/InventoryTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/InvestmentsInPrivatelyHeldCompanies" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/InvestmentsInPrivatelyHeldCompaniesDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/NetIncomePerShare" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/NetIncomePerShareAntiDilutiveSecuritiesDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_CC764D18658E2742A380087472447FBF" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable" xlink:label="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_C2FB301406C18B3D1E21087472432CDD" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_CC764D18658E2742A380087472447FBF" xlink:to="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_C2FB301406C18B3D1E21087472432CDD" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_67759FB6EB8DCF8D37FF08747243379A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_C2FB301406C18B3D1E21087472432CDD" xlink:to="loc_us-gaap_AwardTypeAxis_67759FB6EB8DCF8D37FF08747243379A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_DE48DD57F3BC07756A4608747244E461_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_67759FB6EB8DCF8D37FF08747243379A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_DE48DD57F3BC07756A4608747244E461_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_DE48DD57F3BC07756A4608747244E461" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_67759FB6EB8DCF8D37FF08747243379A" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_DE48DD57F3BC07756A4608747244E461" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="loc_us-gaap_EmployeeStockOptionMember_644C95C7E2EBBB37B35E0874724477F9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_DE48DD57F3BC07756A4608747244E461" xlink:to="loc_us-gaap_EmployeeStockOptionMember_644C95C7E2EBBB37B35E0874724477F9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_716B6D4B0B47D4D4D1DA087472446B02" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_DE48DD57F3BC07756A4608747244E461" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_716B6D4B0B47D4D4D1DA087472446B02" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeStockMember" xlink:label="loc_us-gaap_EmployeeStockMember_6279F59D6675FEDA1F1608747244C80E" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_DE48DD57F3BC07756A4608747244E461" xlink:to="loc_us-gaap_EmployeeStockMember_6279F59D6675FEDA1F1608747244C80E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_4285A47BF79EE06CA83008747244D4BC" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfAntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareTable_C2FB301406C18B3D1E21087472432CDD" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_4285A47BF79EE06CA83008747244D4BC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_7F5BAB0B97194187B20608747244415D_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_4285A47BF79EE06CA83008747244D4BC" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_7F5BAB0B97194187B20608747244415D_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AntidilutiveSecuritiesNameDomain" xlink:label="loc_us-gaap_AntidilutiveSecuritiesNameDomain_7F5BAB0B97194187B20608747244415D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareByAntidilutiveSecuritiesAxis_4285A47BF79EE06CA83008747244D4BC" xlink:to="loc_us-gaap_AntidilutiveSecuritiesNameDomain_7F5BAB0B97194187B20608747244415D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockCompensationPlanMember" xlink:label="loc_us-gaap_StockCompensationPlanMember_5CE0492648BA2748B3F408747244A666" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesNameDomain_7F5BAB0B97194187B20608747244415D" xlink:to="loc_us-gaap_StockCompensationPlanMember_5CE0492648BA2748B3F408747244A666" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount" xlink:label="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_3FC7E5F61FE69AA5748D08747244BAC8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareLineItems_CC764D18658E2742A380087472447FBF" xlink:to="loc_us-gaap_AntidilutiveSecuritiesExcludedFromComputationOfEarningsPerShareAmount_3FC7E5F61FE69AA5748D08747244BAC8" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/NetIncomePerShareCalculationOfBasicAndDilutedDetails" xlink:type="extended">
    <loc xlink:href="ftnt-20160930.xsd#ftnt_EarningsPerShareLineItems" xlink:label="loc_ftnt_EarningsPerShareLineItems_EC5E8EDC93CB72A4454C45331B6C1653" xlink:type="locator" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_ScheduleOfEarningsPerShareTable" xlink:label="loc_ftnt_ScheduleOfEarningsPerShareTable_EC23EB79CC109F226F9745331B6C81EF" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_ftnt_EarningsPerShareLineItems_EC5E8EDC93CB72A4454C45331B6C1653" xlink:to="loc_ftnt_ScheduleOfEarningsPerShareTable_EC23EB79CC109F226F9745331B6C81EF" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_CD034192215B8F9D2C1745331B6C36CC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_ftnt_ScheduleOfEarningsPerShareTable_EC23EB79CC109F226F9745331B6C81EF" xlink:to="loc_us-gaap_AwardTypeAxis_CD034192215B8F9D2C1745331B6C36CC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_F9CA880215DF373A7C3845331B6C71C1_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_CD034192215B8F9D2C1745331B6C36CC" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_F9CA880215DF373A7C3845331B6C71C1_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_F9CA880215DF373A7C3845331B6C71C1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_CD034192215B8F9D2C1745331B6C36CC" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_F9CA880215DF373A7C3845331B6C71C1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="loc_us-gaap_EmployeeStockOptionMember_3F50765F3D32638487D045331B6C1C1D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_F9CA880215DF373A7C3845331B6C71C1" xlink:to="loc_us-gaap_EmployeeStockOptionMember_3F50765F3D32638487D045331B6C1C1D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_3658AB20CE8994FA115E45331B6C747C" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_F9CA880215DF373A7C3845331B6C71C1" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_3658AB20CE8994FA115E45331B6C747C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeStockMember" xlink:label="loc_us-gaap_EmployeeStockMember_8CBA7B24E85D2BD0073545331B6CA78C" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_F9CA880215DF373A7C3845331B6C71C1" xlink:to="loc_us-gaap_EmployeeStockMember_8CBA7B24E85D2BD0073545331B6CA78C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_73BAF97753BB87C5D83945331B6C1D52" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_EC5E8EDC93CB72A4454C45331B6C1653" xlink:to="loc_us-gaap_NetIncomeLoss_73BAF97753BB87C5D83945331B6C1D52" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasicAbstract" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasicAbstract_39BB6BF8A94C2B1B300945331B6CAA37" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_EC5E8EDC93CB72A4454C45331B6C1653" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasicAbstract_39BB6BF8A94C2B1B300945331B6CAA37" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingBasic" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_ACEFFA085B6AE7C55D8845331B6CD8EF" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasicAbstract_39BB6BF8A94C2B1B300945331B6CAA37" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingBasic_ACEFFA085B6AE7C55D8845331B6CD8EF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract" xlink:label="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_BE22D1AEC21D6512C4BF45331B6C4272" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_EC5E8EDC93CB72A4454C45331B6C1653" xlink:to="loc_us-gaap_WeightedAverageNumberOfSharesOutstandingAbstract_BE22D1AEC21D6512C4BF45331B6C4272" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract" xlink:label="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_1E7EE6FE19FA6A0A07F645331B6D5D84" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_EC5E8EDC93CB72A4454C45331B6C1653" xlink:to="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_1E7EE6FE19FA6A0A07F645331B6D5D84" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements" xlink:label="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_F16DFEA8C82C72DF94BA45331B6D6AEE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_1E7EE6FE19FA6A0A07F645331B6D5D84" xlink:to="loc_us-gaap_IncrementalCommonSharesAttributableToShareBasedPaymentArrangements_F16DFEA8C82C72DF94BA45331B6D6AEE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_A0F0DDDBA53CC44D0FED45331B6D883F" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_WeightedAverageNumberDilutedSharesOutstandingAdjustmentAbstract_1E7EE6FE19FA6A0A07F645331B6D5D84" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_A0F0DDDBA53CC44D0FED45331B6D883F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NetIncomeLossAbstract" xlink:label="loc_us-gaap_NetIncomeLossAbstract_C7F12F7B460AEB59B76F45331B6D8711" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EarningsPerShareLineItems_EC5E8EDC93CB72A4454C45331B6C1653" xlink:to="loc_us-gaap_NetIncomeLossAbstract_C7F12F7B460AEB59B76F45331B6D8711" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EarningsPerShareBasic" xlink:label="loc_us-gaap_EarningsPerShareBasic_D6F99EC94950C252095545331B6DD0B2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAbstract_C7F12F7B460AEB59B76F45331B6D8711" xlink:to="loc_us-gaap_EarningsPerShareBasic_D6F99EC94950C252095545331B6DD0B2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EarningsPerShareDiluted" xlink:label="loc_us-gaap_EarningsPerShareDiluted_85A265FB99F7F852467D45331B6D6FAF" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NetIncomeLossAbstract_C7F12F7B460AEB59B76F45331B6D8711" xlink:to="loc_us-gaap_EarningsPerShareDiluted_85A265FB99F7F852467D45331B6D6FAF" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/NetIncomePerShareTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/PropertyAndEquipmentnet" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/PropertyAndEquipmentnetDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentLineItems" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentLineItems_A4FBAED887FD8961F2A60874727C9442" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfPropertyPlantAndEquipmentTable" xlink:label="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_39E0B1417C9CA67C46980874727BC2E2" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_A4FBAED887FD8961F2A60874727C9442" xlink:to="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_39E0B1417C9CA67C46980874727BC2E2" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentByTypeAxis" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_C9484388DC7E2B1E2E300874727BB5E9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfPropertyPlantAndEquipmentTable_39E0B1417C9CA67C46980874727BC2E2" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_C9484388DC7E2B1E2E300874727BB5E9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_E0EDD6C5DA7698F6512A0874727BE975_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_C9484388DC7E2B1E2E300874727BB5E9" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_E0EDD6C5DA7698F6512A0874727BE975_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentTypeDomain" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_E0EDD6C5DA7698F6512A0874727BE975" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentByTypeAxis_C9484388DC7E2B1E2E300874727BB5E9" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_E0EDD6C5DA7698F6512A0874727BE975" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LandMember" xlink:label="loc_us-gaap_LandMember_3905D454C9035A1C64580874727B7BC9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_E0EDD6C5DA7698F6512A0874727BE975" xlink:to="loc_us-gaap_LandMember_3905D454C9035A1C64580874727B7BC9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BuildingAndBuildingImprovementsMember" xlink:label="loc_us-gaap_BuildingAndBuildingImprovementsMember_E0B63CB894121C1CCED80874727BBDC7" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_E0EDD6C5DA7698F6512A0874727BE975" xlink:to="loc_us-gaap_BuildingAndBuildingImprovementsMember_E0B63CB894121C1CCED80874727BBDC7" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_EvaluationUnitsMember" xlink:label="loc_ftnt_EvaluationUnitsMember_05E595478B267B6F76480874727CE592" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_E0EDD6C5DA7698F6512A0874727BE975" xlink:to="loc_ftnt_EvaluationUnitsMember_05E595478B267B6F76480874727CE592" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ComputerEquipmentMember" xlink:label="loc_us-gaap_ComputerEquipmentMember_78AFE837E2A7EEEA51D50874727C367E" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_E0EDD6C5DA7698F6512A0874727BE975" xlink:to="loc_us-gaap_ComputerEquipmentMember_78AFE837E2A7EEEA51D50874727C367E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_FurnitureAndFixturesMember" xlink:label="loc_us-gaap_FurnitureAndFixturesMember_874AFA8F76CD066352B20874727CD6BA" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_E0EDD6C5DA7698F6512A0874727BE975" xlink:to="loc_us-gaap_FurnitureAndFixturesMember_874AFA8F76CD066352B20874727CD6BA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConstructionInProgressMember" xlink:label="loc_us-gaap_ConstructionInProgressMember_5772D0ED65D2A30019BE0874727C6EA4" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_E0EDD6C5DA7698F6512A0874727BE975" xlink:to="loc_us-gaap_ConstructionInProgressMember_5772D0ED65D2A30019BE0874727C6EA4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_LeaseholdsAndLeaseholdImprovementsMember" xlink:label="loc_us-gaap_LeaseholdsAndLeaseholdImprovementsMember_87DE5890D4752DFC274D0874727C1C17" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentTypeDomain_E0EDD6C5DA7698F6512A0874727BE975" xlink:to="loc_us-gaap_LeaseholdsAndLeaseholdImprovementsMember_87DE5890D4752DFC274D0874727C1C17" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract_AF0CB8E8010F513D760E0874727C85AC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_A4FBAED887FD8961F2A60874727C9442" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract_AF0CB8E8010F513D760E0874727C85AC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentGross" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentGross_981836BB52152A8F26C60874727C8882" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract_AF0CB8E8010F513D760E0874727C85AC" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentGross_981836BB52152A8F26C60874727C8882" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment" xlink:label="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_8A18E8AAAE42340A16500874727CACBF" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract_AF0CB8E8010F513D760E0874727C85AC" xlink:to="loc_us-gaap_AccumulatedDepreciationDepletionAndAmortizationPropertyPlantAndEquipment_8A18E8AAAE42340A16500874727CACBF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_5C4267E848B5B1D4268F0874727CD339" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentNetByTypeAbstract_AF0CB8E8010F513D760E0874727C85AC" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_5C4267E848B5B1D4268F0874727CD339" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_Depreciation" xlink:label="loc_us-gaap_Depreciation_94F0FA333F3AB4E669A30874727C3126" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_PropertyPlantAndEquipmentLineItems_A4FBAED887FD8961F2A60874727C9442" xlink:to="loc_us-gaap_Depreciation_94F0FA333F3AB4E669A30874727C3126" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/PropertyAndEquipmentnetTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/RelatedPartyTransactions" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/RelatedPartyTransactionsDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RelatedPartyTransactionLineItems" xlink:label="loc_us-gaap_RelatedPartyTransactionLineItems_34132FA3594B29B2C1912252CFB5FAB4" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable" xlink:label="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_8D7F6A1FCB3E76D8DAF32252CFAF6F0B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_34132FA3594B29B2C1912252CFB5FAB4" xlink:to="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_8D7F6A1FCB3E76D8DAF32252CFAF6F0B" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherCommitmentsAxis" xlink:label="loc_us-gaap_OtherCommitmentsAxis_78B7B84447E94C7F4E0B2252CFB440E9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_8D7F6A1FCB3E76D8DAF32252CFAF6F0B" xlink:to="loc_us-gaap_OtherCommitmentsAxis_78B7B84447E94C7F4E0B2252CFB440E9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherCommitmentsDomain" xlink:label="loc_us-gaap_OtherCommitmentsDomain_C85C5DCA211DA60098292252CFB439AC_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_OtherCommitmentsAxis_78B7B84447E94C7F4E0B2252CFB440E9" xlink:to="loc_us-gaap_OtherCommitmentsDomain_C85C5DCA211DA60098292252CFB439AC_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_OtherCommitmentsDomain" xlink:label="loc_us-gaap_OtherCommitmentsDomain_C85C5DCA211DA60098292252CFB439AC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_OtherCommitmentsAxis_78B7B84447E94C7F4E0B2252CFB440E9" xlink:to="loc_us-gaap_OtherCommitmentsDomain_C85C5DCA211DA60098292252CFB439AC" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_ContingencyFeeArrangementMember" xlink:label="loc_ftnt_ContingencyFeeArrangementMember_34F13DCF5041EA8B572A2252CFB5DA80" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_OtherCommitmentsDomain_C85C5DCA211DA60098292252CFB439AC" xlink:to="loc_ftnt_ContingencyFeeArrangementMember_34F13DCF5041EA8B572A2252CFB5DA80" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RelatedPartyTransactionsByRelatedPartyAxis" xlink:label="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_C9E56DB71857DD3FF0AB2252CFB537D3" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRelatedPartyTransactionsByRelatedPartyTable_8D7F6A1FCB3E76D8DAF32252CFAF6F0B" xlink:to="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_C9E56DB71857DD3FF0AB2252CFB537D3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:label="loc_us-gaap_RelatedPartyDomain_BE39C523FF22F4AF540B2252CFB5AA0A_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_C9E56DB71857DD3FF0AB2252CFB537D3" xlink:to="loc_us-gaap_RelatedPartyDomain_BE39C523FF22F4AF540B2252CFB5AA0A_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RelatedPartyDomain" xlink:label="loc_us-gaap_RelatedPartyDomain_BE39C523FF22F4AF540B2252CFB5AA0A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RelatedPartyTransactionsByRelatedPartyAxis_C9E56DB71857DD3FF0AB2252CFB537D3" xlink:to="loc_us-gaap_RelatedPartyDomain_BE39C523FF22F4AF540B2252CFB5AA0A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ImmediateFamilyMemberOfManagementOrPrincipalOwnerMember" xlink:label="loc_us-gaap_ImmediateFamilyMemberOfManagementOrPrincipalOwnerMember_2D06847548854F6F6F482252CFB5EF06" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyDomain_BE39C523FF22F4AF540B2252CFB5AA0A" xlink:to="loc_us-gaap_ImmediateFamilyMemberOfManagementOrPrincipalOwnerMember_2D06847548854F6F6F482252CFB5EF06" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty" xlink:label="loc_us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty_1C030AEC127FE3537AEA2252CFB502A5" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_34132FA3594B29B2C1912252CFB5FAB4" xlink:to="loc_us-gaap_RelatedPartyTransactionExpensesFromTransactionsWithRelatedParty_1C030AEC127FE3537AEA2252CFB502A5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccountsPayableRelatedPartiesCurrentAndNoncurrent" xlink:label="loc_us-gaap_AccountsPayableRelatedPartiesCurrentAndNoncurrent_7F24A92FAA3F37384A002252CFB5EC2A" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RelatedPartyTransactionLineItems_34132FA3594B29B2C1912252CFB5FAB4" xlink:to="loc_us-gaap_AccountsPayableRelatedPartiesCurrentAndNoncurrent_7F24A92FAA3F37384A002252CFB5EC2A" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/RestructuringCharges" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/RestructuringChargesAdditionalInformationDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringCostAndReserveLineItems" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_C6CE74AEFC8FDB61A413224E28C1CBE1" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_3B0CCBF6CED92D6DBB9B224E28C00AB4" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_C6CE74AEFC8FDB61A413224E28C1CBE1" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_3B0CCBF6CED92D6DBB9B224E28C00AB4" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringPlanAxis" xlink:label="loc_us-gaap_RestructuringPlanAxis_A829E6506AD7496FFBD7224E28C01AC1" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_3B0CCBF6CED92D6DBB9B224E28C00AB4" xlink:to="loc_us-gaap_RestructuringPlanAxis_A829E6506AD7496FFBD7224E28C01AC1" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringPlanDomain" xlink:label="loc_us-gaap_RestructuringPlanDomain_89C36A98DB0206B9D09B224E28C0C978_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringPlanAxis_A829E6506AD7496FFBD7224E28C01AC1" xlink:to="loc_us-gaap_RestructuringPlanDomain_89C36A98DB0206B9D09B224E28C0C978_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringPlanDomain" xlink:label="loc_us-gaap_RestructuringPlanDomain_89C36A98DB0206B9D09B224E28C0C978" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringPlanAxis_A829E6506AD7496FFBD7224E28C01AC1" xlink:to="loc_us-gaap_RestructuringPlanDomain_89C36A98DB0206B9D09B224E28C0C978" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_A2015MeruRestructuringMember" xlink:label="loc_ftnt_A2015MeruRestructuringMember_5A40E41784CD04D75393224E28C0A40D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_89C36A98DB0206B9D09B224E28C0C978" xlink:to="loc_ftnt_A2015MeruRestructuringMember_5A40E41784CD04D75393224E28C0A40D" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_A2016RestructuringMember" xlink:label="loc_ftnt_A2016RestructuringMember_5037835E2630C7E54BBD224E28C08440" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringPlanDomain_89C36A98DB0206B9D09B224E28C0C978" xlink:to="loc_ftnt_A2016RestructuringMember_5037835E2630C7E54BBD224E28C08440" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAxis" xlink:label="loc_us-gaap_BusinessAcquisitionAxis_93B72285BF868927FCCD224E28C02C84" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_3B0CCBF6CED92D6DBB9B224E28C00AB4" xlink:to="loc_us-gaap_BusinessAcquisitionAxis_93B72285BF868927FCCD224E28C02C84" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_F634B72E3C26A2629D31224E28C09673_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_93B72285BF868927FCCD224E28C02C84" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_F634B72E3C26A2629D31224E28C09673_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BusinessAcquisitionAcquireeDomain" xlink:label="loc_us-gaap_BusinessAcquisitionAcquireeDomain_F634B72E3C26A2629D31224E28C09673" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BusinessAcquisitionAxis_93B72285BF868927FCCD224E28C02C84" xlink:to="loc_us-gaap_BusinessAcquisitionAcquireeDomain_F634B72E3C26A2629D31224E28C09673" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_MeruNetworksInc.Member" xlink:label="loc_ftnt_MeruNetworksInc.Member_9068178172407ABB2D0E224E28C0C6E7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BusinessAcquisitionAcquireeDomain_F634B72E3C26A2629D31224E28C09673" xlink:to="loc_ftnt_MeruNetworksInc.Member_9068178172407ABB2D0E224E28C0C6E7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationAxis" xlink:label="loc_us-gaap_BalanceSheetLocationAxis_A6BE24517C148C7A23E7224E28C00029" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_3B0CCBF6CED92D6DBB9B224E28C00AB4" xlink:to="loc_us-gaap_BalanceSheetLocationAxis_A6BE24517C148C7A23E7224E28C00029" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_79F87E626A99BA93AA4C224E28C1C712_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_A6BE24517C148C7A23E7224E28C00029" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_79F87E626A99BA93AA4C224E28C1C712_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_BalanceSheetLocationDomain" xlink:label="loc_us-gaap_BalanceSheetLocationDomain_79F87E626A99BA93AA4C224E28C1C712" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_BalanceSheetLocationAxis_A6BE24517C148C7A23E7224E28C00029" xlink:to="loc_us-gaap_BalanceSheetLocationDomain_79F87E626A99BA93AA4C224E28C1C712" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccruedLiabilitiesMember" xlink:label="loc_us-gaap_AccruedLiabilitiesMember_591EBCCFD9466D9FDD9F224E28C1E5DE" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_BalanceSheetLocationDomain_79F87E626A99BA93AA4C224E28C1C712" xlink:to="loc_us-gaap_AccruedLiabilitiesMember_591EBCCFD9466D9FDD9F224E28C1E5DE" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringAndRelatedCostExpectedCost1" xlink:label="loc_us-gaap_RestructuringAndRelatedCostExpectedCost1_A4C9E7EEC45C22E198EB224E28C1F286" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_C6CE74AEFC8FDB61A413224E28C1CBE1" xlink:to="loc_us-gaap_RestructuringAndRelatedCostExpectedCost1_A4C9E7EEC45C22E198EB224E28C1F286" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringAndRelatedCostCostIncurredToDate1" xlink:label="loc_us-gaap_RestructuringAndRelatedCostCostIncurredToDate1_59DE4340ED6132CA6C51224E28C11887" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_C6CE74AEFC8FDB61A413224E28C1CBE1" xlink:to="loc_us-gaap_RestructuringAndRelatedCostCostIncurredToDate1_59DE4340ED6132CA6C51224E28C11887" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringReserve" xlink:label="loc_us-gaap_RestructuringReserve_09B18CD4D9C48D3C64B7224E28C1BB4C" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_C6CE74AEFC8FDB61A413224E28C1CBE1" xlink:to="loc_us-gaap_RestructuringReserve_09B18CD4D9C48D3C64B7224E28C1BB4C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringCharges" xlink:label="loc_us-gaap_RestructuringCharges_49E28BE6150D6044432C224E28C1A1C4" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_C6CE74AEFC8FDB61A413224E28C1CBE1" xlink:to="loc_us-gaap_RestructuringCharges_49E28BE6150D6044432C224E28C1A1C4" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/RestructuringChargesRestructuringActivityDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringCostAndReserveLineItems" xlink:label="loc_us-gaap_RestructuringCostAndReserveLineItems_6313F5E4B5302418F2FD80A2AB984E9C" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfRestructuringAndRelatedCostsTable" xlink:label="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_053C88640271803ECFEF80A2AB98542E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_6313F5E4B5302418F2FD80A2AB984E9C" xlink:to="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_053C88640271803ECFEF80A2AB98542E" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringCostAndReserveAxis" xlink:label="loc_us-gaap_RestructuringCostAndReserveAxis_47AE6370F5F99491822480A2AB98871D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfRestructuringAndRelatedCostsTable_053C88640271803ECFEF80A2AB98542E" xlink:to="loc_us-gaap_RestructuringCostAndReserveAxis_47AE6370F5F99491822480A2AB98871D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_456058F794A538D513D480A2AB985B08_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_47AE6370F5F99491822480A2AB98871D" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_456058F794A538D513D480A2AB985B08_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TypeOfRestructuringDomain" xlink:label="loc_us-gaap_TypeOfRestructuringDomain_456058F794A538D513D480A2AB985B08" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RestructuringCostAndReserveAxis_47AE6370F5F99491822480A2AB98871D" xlink:to="loc_us-gaap_TypeOfRestructuringDomain_456058F794A538D513D480A2AB985B08" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeSeveranceMember" xlink:label="loc_us-gaap_EmployeeSeveranceMember_6980C7AC026FC78F099180A2AB98F000" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_456058F794A538D513D480A2AB985B08" xlink:to="loc_us-gaap_EmployeeSeveranceMember_6980C7AC026FC78F099180A2AB98F000" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ContractTerminationMember" xlink:label="loc_us-gaap_ContractTerminationMember_6E7162696662ABF14C4380A2AB98AF0E" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_TypeOfRestructuringDomain_456058F794A538D513D480A2AB985B08" xlink:to="loc_us-gaap_ContractTerminationMember_6E7162696662ABF14C4380A2AB98AF0E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringReserve" xlink:label="loc_us-gaap_RestructuringReserve_6B458C9E1F3AB9E7CAA780A2AB988D77" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_6313F5E4B5302418F2FD80A2AB984E9C" xlink:to="loc_us-gaap_RestructuringReserve_6B458C9E1F3AB9E7CAA780A2AB988D77" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringCharges" xlink:label="loc_us-gaap_RestructuringCharges_A8B26098511BC6855D7680A2AB992BC8" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_6313F5E4B5302418F2FD80A2AB984E9C" xlink:to="loc_us-gaap_RestructuringCharges_A8B26098511BC6855D7680A2AB992BC8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PaymentsForRestructuring" xlink:label="loc_us-gaap_PaymentsForRestructuring_EAB4C476F99659B8996680A2AB994DFC" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_6313F5E4B5302418F2FD80A2AB984E9C" xlink:to="loc_us-gaap_PaymentsForRestructuring_EAB4C476F99659B8996680A2AB994DFC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestructuringReserveSettledWithoutCash1" xlink:label="loc_us-gaap_RestructuringReserveSettledWithoutCash1_93D0D7ABDF932DD3D72980A2AB99CF69" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RestructuringCostAndReserveLineItems_6313F5E4B5302418F2FD80A2AB984E9C" xlink:to="loc_us-gaap_RestructuringReserveSettledWithoutCash1_93D0D7ABDF932DD3D72980A2AB99CF69" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/RestructuringChargesTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/SegmentAndSignificantCustomerInformation" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/SegmentAndSignificantCustomerInformationDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentReportingInformationLineItems" xlink:label="loc_us-gaap_SegmentReportingInformationLineItems_F374A4B3600744D52C2F80A2AD672BF7" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable" xlink:label="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_71C6022BD53FBA80162080A2AD65E069" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_F374A4B3600744D52C2F80A2AD672BF7" xlink:to="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_71C6022BD53FBA80162080A2AD65E069" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementGeographicalAxis" xlink:label="loc_us-gaap_StatementGeographicalAxis_04B19252DCCA469188EC80A2AD65BE67" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfSegmentReportingInformationBySegmentTable_71C6022BD53FBA80162080A2AD65E069" xlink:to="loc_us-gaap_StatementGeographicalAxis_04B19252DCCA469188EC80A2AD65BE67" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_C88540D38CAD0626513080A2AD669B53_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementGeographicalAxis_04B19252DCCA469188EC80A2AD65BE67" xlink:to="loc_us-gaap_SegmentGeographicalDomain_C88540D38CAD0626513080A2AD669B53_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SegmentGeographicalDomain" xlink:label="loc_us-gaap_SegmentGeographicalDomain_C88540D38CAD0626513080A2AD669B53" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementGeographicalAxis_04B19252DCCA469188EC80A2AD65BE67" xlink:to="loc_us-gaap_SegmentGeographicalDomain_C88540D38CAD0626513080A2AD669B53" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AmericasMember" xlink:label="loc_us-gaap_AmericasMember_9E5A432648F05595C48F80A2AD66DE5B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_C88540D38CAD0626513080A2AD669B53" xlink:to="loc_us-gaap_AmericasMember_9E5A432648F05595C48F80A2AD66DE5B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/country/2013/country-2013-01-31.xsd#country_US" xlink:label="loc_country_US_70F8AC7A2E914031091D80A2AD660A3C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AmericasMember_9E5A432648F05595C48F80A2AD66DE5B" xlink:to="loc_country_US_70F8AC7A2E914031091D80A2AD660A3C" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_CanadaMember" xlink:label="loc_ftnt_CanadaMember_283F134047C2ECB183D480A2AD66BB2D" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AmericasMember_9E5A432648F05595C48F80A2AD66DE5B" xlink:to="loc_ftnt_CanadaMember_283F134047C2ECB183D480A2AD66BB2D" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_OtherAmericasMember" xlink:label="loc_ftnt_OtherAmericasMember_E3B28E76D458555CAA4E80A2AD661270" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_AmericasMember_9E5A432648F05595C48F80A2AD66DE5B" xlink:to="loc_ftnt_OtherAmericasMember_E3B28E76D458555CAA4E80A2AD661270" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_EuropeMiddleEastAndAfricaMember" xlink:label="loc_ftnt_EuropeMiddleEastAndAfricaMember_E84BE3DE3FB53413AD6080A2AD66864E" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_C88540D38CAD0626513080A2AD669B53" xlink:to="loc_ftnt_EuropeMiddleEastAndAfricaMember_E84BE3DE3FB53413AD6080A2AD66864E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/country/2013/country-2013-01-31.xsd#country_FR" xlink:label="loc_country_FR_877F13E41EE60F92DEAF80A2AD668FA8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EuropeMiddleEastAndAfricaMember_E84BE3DE3FB53413AD6080A2AD66864E" xlink:to="loc_country_FR_877F13E41EE60F92DEAF80A2AD668FA8" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_OtherEuropeMiddleEastandAfricanCountriesMember" xlink:label="loc_ftnt_OtherEuropeMiddleEastandAfricanCountriesMember_24B2F82460BE25EA68AA80A2AD67A429" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_ftnt_EuropeMiddleEastAndAfricaMember_E84BE3DE3FB53413AD6080A2AD66864E" xlink:to="loc_ftnt_OtherEuropeMiddleEastandAfricanCountriesMember_24B2F82460BE25EA68AA80A2AD67A429" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_AsiaPacificAndJapanMember" xlink:label="loc_ftnt_AsiaPacificAndJapanMember_0597279ABC319AAF973A80A2AD67EEF3" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentGeographicalDomain_C88540D38CAD0626513080A2AD669B53" xlink:to="loc_ftnt_AsiaPacificAndJapanMember_0597279ABC319AAF973A80A2AD67EEF3" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_NumberOfBusinessActivities" xlink:label="loc_ftnt_NumberOfBusinessActivities_E7E6FEBCD38FED3DDA0C80A2AD672CBD" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_F374A4B3600744D52C2F80A2AD672BF7" xlink:to="loc_ftnt_NumberOfBusinessActivities_E7E6FEBCD38FED3DDA0C80A2AD672CBD" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_NumberOfSegmentManagersResponsibleForOperations" xlink:label="loc_ftnt_NumberOfSegmentManagersResponsibleForOperations_D3989D7F93450181BA8780A2AD67E229" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_F374A4B3600744D52C2F80A2AD672BF7" xlink:to="loc_ftnt_NumberOfSegmentManagersResponsibleForOperations_D3989D7F93450181BA8780A2AD67E229" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NumberOfOperatingSegments" xlink:label="loc_us-gaap_NumberOfOperatingSegments_B453A0772B8B4C29400D80A2AD679EFC" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_F374A4B3600744D52C2F80A2AD672BF7" xlink:to="loc_us-gaap_NumberOfOperatingSegments_B453A0772B8B4C29400D80A2AD679EFC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NumberOfReportableSegments" xlink:label="loc_us-gaap_NumberOfReportableSegments_2368F150347C4CB7201980A2AD679A7C" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_F374A4B3600744D52C2F80A2AD672BF7" xlink:to="loc_us-gaap_NumberOfReportableSegments_2368F150347C4CB7201980A2AD679A7C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SalesRevenueNet" xlink:label="loc_us-gaap_SalesRevenueNet_17AD54438C916E93802680A2AD673AC5" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_F374A4B3600744D52C2F80A2AD672BF7" xlink:to="loc_us-gaap_SalesRevenueNet_17AD54438C916E93802680A2AD673AC5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PropertyPlantAndEquipmentNet" xlink:label="loc_us-gaap_PropertyPlantAndEquipmentNet_E7E4A8DD4E8C88F06F6F80A2AD67C33C" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SegmentReportingInformationLineItems_F374A4B3600744D52C2F80A2AD672BF7" xlink:to="loc_us-gaap_PropertyPlantAndEquipmentNet_E7E4A8DD4E8C88F06F6F80A2AD67C33C" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/SegmentAndSignificantCustomerInformationSignificantCustomersDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EntityWideRevenueMajorCustomerLineItems" xlink:label="loc_us-gaap_EntityWideRevenueMajorCustomerLineItems_224179FDB4A2039CCC292C634E67BB42" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable" xlink:label="loc_us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable_CBA66F5D5741606571F62C634E64F845" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EntityWideRevenueMajorCustomerLineItems_224179FDB4A2039CCC292C634E67BB42" xlink:to="loc_us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable_CBA66F5D5741606571F62C634E64F845" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_LegalEntityAxis" xlink:label="loc_dei_LegalEntityAxis_4A731E6114E857C148B12C634E658363" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable_CBA66F5D5741606571F62C634E64F845" xlink:to="loc_dei_LegalEntityAxis_4A731E6114E857C148B12C634E658363" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_BD0970A05EE42AC75DE42C634E657D0D_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_dei_LegalEntityAxis_4A731E6114E857C148B12C634E658363" xlink:to="loc_dei_EntityDomain_BD0970A05EE42AC75DE42C634E657D0D_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.sec.gov/dei/2014/dei-2014-01-31.xsd#dei_EntityDomain" xlink:label="loc_dei_EntityDomain_BD0970A05EE42AC75DE42C634E657D0D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_dei_LegalEntityAxis_4A731E6114E857C148B12C634E658363" xlink:to="loc_dei_EntityDomain_BD0970A05EE42AC75DE42C634E657D0D" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_ExclusiveNetworksGroupMember" xlink:label="loc_ftnt_ExclusiveNetworksGroupMember_50897ECAAB2EB02F96AE2C634E65D15F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_BD0970A05EE42AC75DE42C634E657D0D" xlink:to="loc_ftnt_ExclusiveNetworksGroupMember_50897ECAAB2EB02F96AE2C634E65D15F" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_FinTecComputersMember" xlink:label="loc_ftnt_FinTecComputersMember_375FE955A2A00F0A7B672C634E6590A6" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_BD0970A05EE42AC75DE42C634E657D0D" xlink:to="loc_ftnt_FinTecComputersMember_375FE955A2A00F0A7B672C634E6590A6" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_IngramMicroMember" xlink:label="loc_ftnt_IngramMicroMember_7569BDA4BA6FC6999A132C634E659C50" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_dei_EntityDomain_BD0970A05EE42AC75DE42C634E657D0D" xlink:to="loc_ftnt_IngramMicroMember_7569BDA4BA6FC6999A132C634E659C50" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConcentrationRiskByTypeAxis" xlink:label="loc_us-gaap_ConcentrationRiskByTypeAxis_FF3A19DC4EEBD21351672C634E666533" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable_CBA66F5D5741606571F62C634E64F845" xlink:to="loc_us-gaap_ConcentrationRiskByTypeAxis_FF3A19DC4EEBD21351672C634E666533" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConcentrationRiskTypeDomain" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_8B2455EA9C21959B3D442C634E662575_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_FF3A19DC4EEBD21351672C634E666533" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_8B2455EA9C21959B3D442C634E662575_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConcentrationRiskTypeDomain" xlink:label="loc_us-gaap_ConcentrationRiskTypeDomain_8B2455EA9C21959B3D442C634E662575" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByTypeAxis_FF3A19DC4EEBD21351672C634E666533" xlink:to="loc_us-gaap_ConcentrationRiskTypeDomain_8B2455EA9C21959B3D442C634E662575" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CustomerConcentrationRiskMember" xlink:label="loc_us-gaap_CustomerConcentrationRiskMember_D02D3CC602FEFFA19CA42C634E668893" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskTypeDomain_8B2455EA9C21959B3D442C634E662575" xlink:to="loc_us-gaap_CustomerConcentrationRiskMember_D02D3CC602FEFFA19CA42C634E668893" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConcentrationRiskByBenchmarkAxis" xlink:label="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_DCE6928325EDFF613F9C2C634E66F081" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEntityWideRevenueByMajorCustomersByReportingSegmentsTable_CBA66F5D5741606571F62C634E64F845" xlink:to="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_DCE6928325EDFF613F9C2C634E66F081" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConcentrationRiskBenchmarkDomain" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_CCACE8FD262E1BD34CC52C634E660DB7_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_DCE6928325EDFF613F9C2C634E66F081" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_CCACE8FD262E1BD34CC52C634E660DB7_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConcentrationRiskBenchmarkDomain" xlink:label="loc_us-gaap_ConcentrationRiskBenchmarkDomain_CCACE8FD262E1BD34CC52C634E660DB7" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ConcentrationRiskByBenchmarkAxis_DCE6928325EDFF613F9C2C634E66F081" xlink:to="loc_us-gaap_ConcentrationRiskBenchmarkDomain_CCACE8FD262E1BD34CC52C634E660DB7" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SalesRevenueNetMember" xlink:label="loc_us-gaap_SalesRevenueNetMember_F1E7B716FCACF9B3D2152C634E669D45" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_CCACE8FD262E1BD34CC52C634E660DB7" xlink:to="loc_us-gaap_SalesRevenueNetMember_F1E7B716FCACF9B3D2152C634E669D45" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AccountsReceivableMember" xlink:label="loc_us-gaap_AccountsReceivableMember_4DCBEBB912618E253E162C634E676E4C" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ConcentrationRiskBenchmarkDomain_CCACE8FD262E1BD34CC52C634E660DB7" xlink:to="loc_us-gaap_AccountsReceivableMember_4DCBEBB912618E253E162C634E676E4C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ConcentrationRiskPercentage1" xlink:label="loc_us-gaap_ConcentrationRiskPercentage1_343098FB1F04875550ED2C634E67E893" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EntityWideRevenueMajorCustomerLineItems_224179FDB4A2039CCC292C634E67BB42" xlink:to="loc_us-gaap_ConcentrationRiskPercentage1_343098FB1F04875550ED2C634E67E893" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/SegmentAndSignificantCustomerInformationTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/StockholdersEquity" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/StockholdersEquityAllocationOfStockBasedCompensationExpenseDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_BAF7330997BE26695D674531079CF57C" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable" xlink:label="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_9EA43D34D86DF73F59134531079B9F1F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_BAF7330997BE26695D674531079CF57C" xlink:to="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_9EA43D34D86DF73F59134531079B9F1F" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_0EBEC12282C89473C7F74531079BB980" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_9EA43D34D86DF73F59134531079B9F1F" xlink:to="loc_us-gaap_AwardTypeAxis_0EBEC12282C89473C7F74531079BB980" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_16B335FA9B689ACA591C4531079CA4EF_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_0EBEC12282C89473C7F74531079BB980" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_16B335FA9B689ACA591C4531079CA4EF_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_16B335FA9B689ACA591C4531079CA4EF" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_0EBEC12282C89473C7F74531079BB980" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_16B335FA9B689ACA591C4531079CA4EF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_AAD811316CE4E8E16E184531079C1F41" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_16B335FA9B689ACA591C4531079CA4EF" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_AAD811316CE4E8E16E184531079C1F41" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="loc_us-gaap_EmployeeStockOptionMember_FA0B3BD7C2F6E288E7BB4531079C678E" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_16B335FA9B689ACA591C4531079CA4EF" xlink:to="loc_us-gaap_EmployeeStockOptionMember_FA0B3BD7C2F6E288E7BB4531079C678E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeStockMember" xlink:label="loc_us-gaap_EmployeeStockMember_55EABFF09A832B2973154531079CEB83" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_16B335FA9B689ACA591C4531079CA4EF" xlink:to="loc_us-gaap_EmployeeStockMember_55EABFF09A832B2973154531079CEB83" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationAxis" xlink:label="loc_us-gaap_IncomeStatementLocationAxis_16137BE37DBC580C53084531079CE96B" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfEmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsTable_9EA43D34D86DF73F59134531079B9F1F" xlink:to="loc_us-gaap_IncomeStatementLocationAxis_16137BE37DBC580C53084531079CE96B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_7C853E853750978060514531079C8003_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_16137BE37DBC580C53084531079CE96B" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_7C853E853750978060514531079C8003_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeStatementLocationDomain" xlink:label="loc_us-gaap_IncomeStatementLocationDomain_7C853E853750978060514531079C8003" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_IncomeStatementLocationAxis_16137BE37DBC580C53084531079CE96B" xlink:to="loc_us-gaap_IncomeStatementLocationDomain_7C853E853750978060514531079C8003" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_CostOfGoodsSoldMember" xlink:label="loc_ftnt_CostOfGoodsSoldMember_B0DC5C8DD342EB4EFB464531079C56CC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_7C853E853750978060514531079C8003" xlink:to="loc_ftnt_CostOfGoodsSoldMember_B0DC5C8DD342EB4EFB464531079C56CC" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_CostOfServicesMember" xlink:label="loc_ftnt_CostOfServicesMember_68E70C2A069325E47DC84531079CDEDF" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_7C853E853750978060514531079C8003" xlink:to="loc_ftnt_CostOfServicesMember_68E70C2A069325E47DC84531079CDEDF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ResearchAndDevelopmentExpenseMember" xlink:label="loc_us-gaap_ResearchAndDevelopmentExpenseMember_F535A1B8B34DE4DDC5BE4531079C514B" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_7C853E853750978060514531079C8003" xlink:to="loc_us-gaap_ResearchAndDevelopmentExpenseMember_F535A1B8B34DE4DDC5BE4531079C514B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SellingAndMarketingExpenseMember" xlink:label="loc_us-gaap_SellingAndMarketingExpenseMember_062704DD057B27243BEF4531079C816A" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_7C853E853750978060514531079C8003" xlink:to="loc_us-gaap_SellingAndMarketingExpenseMember_062704DD057B27243BEF4531079C816A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_GeneralAndAdministrativeExpenseMember" xlink:label="loc_us-gaap_GeneralAndAdministrativeExpenseMember_7E2A8D611E270724E90F4531079CD94B" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_IncomeStatementLocationDomain_7C853E853750978060514531079C8003" xlink:to="loc_us-gaap_GeneralAndAdministrativeExpenseMember_7E2A8D611E270724E90F4531079CD94B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_BFBCD9E7CB160B02EC524531079CAD97" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_BAF7330997BE26695D674531079CF57C" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_BFBCD9E7CB160B02EC524531079CAD97" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense_C43709AAAB2A161AA4564531079CCA3A" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EmployeeServiceShareBasedCompensationAllocationOfRecognizedPeriodCostsLineItems_BAF7330997BE26695D674531079CF57C" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationTaxBenefitFromCompensationExpense_C43709AAAB2A161AA4564531079CCA3A" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/StockholdersEquityEmployeeStockOptionsActivityDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_E6C048E3974B2DD736994531064BA521" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_0D5888EDDD6CF73492644531064BEC8C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_E6C048E3974B2DD736994531064BA521" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_0D5888EDDD6CF73492644531064BEC8C" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_0CABE2F065934FF5AFB94531064B22A0" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_0D5888EDDD6CF73492644531064BEC8C" xlink:to="loc_us-gaap_AwardTypeAxis_0CABE2F065934FF5AFB94531064B22A0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4AFEA27A04C13A5D5C524531064BEA55_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_0CABE2F065934FF5AFB94531064B22A0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4AFEA27A04C13A5D5C524531064BEA55_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4AFEA27A04C13A5D5C524531064BEA55" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_0CABE2F065934FF5AFB94531064B22A0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4AFEA27A04C13A5D5C524531064BEA55" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeStockOptionMember" xlink:label="loc_us-gaap_EmployeeStockOptionMember_11A23B42FFDCF9D470524531064B304D" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_4AFEA27A04C13A5D5C524531064BEA55" xlink:to="loc_us-gaap_EmployeeStockOptionMember_11A23B42FFDCF9D470524531064B304D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_854BD7DD6863DF13625C4531064C53AA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_E6C048E3974B2DD736994531064BA521" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_854BD7DD6863DF13625C4531064C53AA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_1393955B46CEF9D71FB74531064C8FCC" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_854BD7DD6863DF13625C4531064C53AA" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_1393955B46CEF9D71FB74531064C8FCC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_F86EDC14EC612759A9B34531064CB7CC" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_854BD7DD6863DF13625C4531064C53AA" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_F86EDC14EC612759A9B34531064CB7CC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_60EDDF289D23EA7B1F6A4531064CFA67" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_854BD7DD6863DF13625C4531064C53AA" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_60EDDF289D23EA7B1F6A4531064CFA67" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_C03DEF5E2A2B7AD717894531064C78EF" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsAndMethodologyAbstract_854BD7DD6863DF13625C4531064C53AA" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_C03DEF5E2A2B7AD717894531064C78EF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_420B01D77CD2B229CE7D4531064C0C20" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_E6C048E3974B2DD736994531064BA521" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_420B01D77CD2B229CE7D4531064C0C20" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_E2DFB5EA7D12E0C10F7E4531064CE83A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_420B01D77CD2B229CE7D4531064C0C20" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingNumber_E2DFB5EA7D12E0C10F7E4531064CE83A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_2B1943D59D4C5E6743BF4531064C06A3" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_420B01D77CD2B229CE7D4531064C0C20" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriod_2B1943D59D4C5E6743BF4531064C06A3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_CEC73A35AF60967F0D824531064C1635" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_420B01D77CD2B229CE7D4531064C0C20" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsForfeituresInPeriod_CEC73A35AF60967F0D824531064C1635" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_7AA2647076F543321E6C4531064C9B03" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingRollForward_420B01D77CD2B229CE7D4531064C0C20" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesStockOptionsExercised_7AA2647076F543321E6C4531064C9B03" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_45C081CDCE9B4243B9C04531064C2EAC" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_E6C048E3974B2DD736994531064BA521" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_45C081CDCE9B4243B9C04531064C2EAC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_D63C00309AA8B7A7E6DC4531064DD44E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_45C081CDCE9B4243B9C04531064C2EAC" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePrice_D63C00309AA8B7A7E6DC4531064DD44E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_F8D54171C455E1E396394531064D334C" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_45C081CDCE9B4243B9C04531064C2EAC" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageExercisePrice_F8D54171C455E1E396394531064D334C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_6C2A63F84F886EDC4A9F4531064DAD0D" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_45C081CDCE9B4243B9C04531064C2EAC" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsForfeituresInPeriodWeightedAverageExercisePrice_6C2A63F84F886EDC4A9F4531064DAD0D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_276D2AAC0105ABA981B54531064DF14E" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingWeightedAverageExercisePriceRollforward_45C081CDCE9B4243B9C04531064C2EAC" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardOptionsExercisesInPeriodWeightedAverageExercisePrice_276D2AAC0105ABA981B54531064DF14E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_43E9669EE03C7C6F15CB4531064D616D" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_E6C048E3974B2DD736994531064BA521" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_43E9669EE03C7C6F15CB4531064D616D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_6FA9ECBAA74FA4FDA79F4531064D60E6" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_43E9669EE03C7C6F15CB4531064D616D" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsOutstandingWeightedAverageRemainingContractualTerm2_6FA9ECBAA74FA4FDA79F4531064D60E6" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_DB6F1C24772D62C70C714531064E9669" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_43E9669EE03C7C6F15CB4531064D616D" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsOutstandingIntrinsicValue_DB6F1C24772D62C70C714531064E9669" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_A5E5CD13BDF9F256808A4531064ED9B9" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_43E9669EE03C7C6F15CB4531064D616D" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableNumber_A5E5CD13BDF9F256808A4531064ED9B9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_97C65B59FA73C3F5AF594531064ECC54" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_43E9669EE03C7C6F15CB4531064D616D" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisableWeightedAverageExercisePrice_97C65B59FA73C3F5AF594531064ECC54" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_6A6A15FBD1D66CD779574531064E83D0" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_43E9669EE03C7C6F15CB4531064D616D" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableWeightedAverageRemainingContractualTerm1_6A6A15FBD1D66CD779574531064E83D0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_50DD708AA8CC0FE13B4F4531064E7B69" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsAdditionalDisclosuresAbstract_43E9669EE03C7C6F15CB4531064D616D" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsExercisableIntrinsicValue1_50DD708AA8CC0FE13B4F4531064E7B69" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_4C98DCF60E305A1F805F4531064EA467" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_E6C048E3974B2DD736994531064BA521" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_4C98DCF60E305A1F805F4531064EA467" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_B6CA8D789D87AB3798B24531064E3E3B" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_E6C048E3974B2DD736994531064BA521" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_B6CA8D789D87AB3798B24531064E3E3B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_523B65CD7F0469D2F4C34531064EAE23" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_E6C048E3974B2DD736994531064BA521" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_523B65CD7F0469D2F4C34531064EAE23" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_B95F87A33AA2D56194514531064E7BD5" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_E6C048E3974B2DD736994531064BA521" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardOptionsExercisesInPeriodTotalIntrinsicValue_B95F87A33AA2D56194514531064E7BD5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_79438D55218355C259C34531064EF5A8" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_E6C048E3974B2DD736994531064BA521" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardOptionsVestedInPeriodFairValue1_79438D55218355C259C34531064EF5A8" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/StockholdersEquityEsppInformationDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_D1F4955E39416C6A7E4330150C9132A4" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_8C133AD5D94AC85D612E30150C8BAF97" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_D1F4955E39416C6A7E4330150C9132A4" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_8C133AD5D94AC85D612E30150C8BAF97" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_09AE93FE8A8746EDC6C930150C919ECB" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_8C133AD5D94AC85D612E30150C8BAF97" xlink:to="loc_us-gaap_AwardTypeAxis_09AE93FE8A8746EDC6C930150C919ECB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_27B6025A84C54ED65BF430150C913F01_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_09AE93FE8A8746EDC6C930150C919ECB" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_27B6025A84C54ED65BF430150C913F01_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_27B6025A84C54ED65BF430150C913F01" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_09AE93FE8A8746EDC6C930150C919ECB" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_27B6025A84C54ED65BF430150C913F01" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_EmployeeStockPurchasePlanMember" xlink:label="loc_ftnt_EmployeeStockPurchasePlanMember_9CFF74E4CEBC599BCA4930150C91D4E3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_27B6025A84C54ED65BF430150C913F01" xlink:to="loc_ftnt_EmployeeStockPurchasePlanMember_9CFF74E4CEBC599BCA4930150C91D4E3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1" xlink:label="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_E9BA3E122FBAE5020B9C30150C919D6F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_D1F4955E39416C6A7E4330150C9132A4" xlink:to="loc_us-gaap_SharebasedCompensationArrangementBySharebasedPaymentAwardFairValueAssumptionsExpectedTerm1_E9BA3E122FBAE5020B9C30150C919D6F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_731280D7BE43B6E8587B30150C91F5EC" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_D1F4955E39416C6A7E4330150C9132A4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedVolatilityRate_731280D7BE43B6E8587B30150C91F5EC" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_F0CCC91E881076ACCA6830150C92EB16" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_D1F4955E39416C6A7E4330150C9132A4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsRiskFreeInterestRate_F0CCC91E881076ACCA6830150C92EB16" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_A842646A40F2039DAAEC30150C92308F" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_D1F4955E39416C6A7E4330150C9132A4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardFairValueAssumptionsExpectedDividendRate_A842646A40F2039DAAEC30150C92308F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_C81072504EDB4A16A71430150C926162" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_D1F4955E39416C6A7E4330150C9132A4" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_C81072504EDB4A16A71430150C926162" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans" xlink:label="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_8F4D979D0AEBC100110630150C92D0F9" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_D1F4955E39416C6A7E4330150C9132A4" xlink:to="loc_us-gaap_StockIssuedDuringPeriodSharesEmployeeStockPurchasePlans_8F4D979D0AEBC100110630150C92D0F9" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsIssuedWeightedAveragePricePerShare" xlink:label="loc_ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsIssuedWeightedAveragePricePerShare_A81611A84819501B1EAC30150C922996" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_D1F4955E39416C6A7E4330150C9132A4" xlink:to="loc_ftnt_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsIssuedWeightedAveragePricePerShare_A81611A84819501B1EAC30150C922996" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/StockholdersEquityRestrictedStockUnitsActivityDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C485CDD00A6A8C9BE2B9224C2ABC47ED" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_CD4732FB7B7747920118224C2ABB27D8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C485CDD00A6A8C9BE2B9224C2ABC47ED" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_CD4732FB7B7747920118224C2ABB27D8" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AwardTypeAxis" xlink:label="loc_us-gaap_AwardTypeAxis_2E8903126B3BA75A2FEF224C2ABB3A51" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_CD4732FB7B7747920118224C2ABB27D8" xlink:to="loc_us-gaap_AwardTypeAxis_2E8903126B3BA75A2FEF224C2ABB3A51" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_AFCA6EF628BDFA29920E224C2ABCA3BA_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_AwardTypeAxis_2E8903126B3BA75A2FEF224C2ABB3A51" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_AFCA6EF628BDFA29920E224C2ABCA3BA_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_AFCA6EF628BDFA29920E224C2ABCA3BA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_AwardTypeAxis_2E8903126B3BA75A2FEF224C2ABB3A51" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_AFCA6EF628BDFA29920E224C2ABCA3BA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RestrictedStockUnitsRSUMember" xlink:label="loc_us-gaap_RestrictedStockUnitsRSUMember_6CD1400D8267406FDE61224C2ABC26CB" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_AFCA6EF628BDFA29920E224C2ABCA3BA" xlink:to="loc_us-gaap_RestrictedStockUnitsRSUMember_6CD1400D8267406FDE61224C2ABC26CB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PerformanceSharesMember" xlink:label="loc_us-gaap_PerformanceSharesMember_66501530688F6369621B224C2ABCBA7E" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementsByShareBasedPaymentAwardAwardTypeAndPlanNameDomain_AFCA6EF628BDFA29920E224C2ABCA3BA" xlink:to="loc_us-gaap_PerformanceSharesMember_66501530688F6369621B224C2ABCBA7E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_2BDE2629E90F05A82C73224C2ABCC3A0" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C485CDD00A6A8C9BE2B9224C2ABC47ED" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_2BDE2629E90F05A82C73224C2ABCC3A0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_4749F7A412D2AAC781E5224C2ABC119C" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_2BDE2629E90F05A82C73224C2ABCC3A0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedNumber_4749F7A412D2AAC781E5224C2ABC119C" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_761196DC197981DFF747224C2ABCE315" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_2BDE2629E90F05A82C73224C2ABCC3A0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriod_761196DC197981DFF747224C2ABCE315" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_812CFC5C90739D6B0AB7224C2ABCDF67" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_2BDE2629E90F05A82C73224C2ABCC3A0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeitedInPeriod_812CFC5C90739D6B0AB7224C2ABCDF67" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_66E2CA853787B62A920F224C2ABCBB2B" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedRollForward_2BDE2629E90F05A82C73224C2ABCC3A0" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriod_66E2CA853787B62A920F224C2ABCBB2B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_D187BFCE52CD217281C5224C2ABC929F" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C485CDD00A6A8C9BE2B9224C2ABC47ED" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_D187BFCE52CD217281C5224C2ABC929F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_3400CF258BAD3590D979224C2ABCB7B8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_D187BFCE52CD217281C5224C2ABC929F" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValue_3400CF258BAD3590D979224C2ABCB7B8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_0F9BC2D6ED138B660036224C2ABCAE32" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_D187BFCE52CD217281C5224C2ABC929F" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsGrantsInPeriodWeightedAverageGrantDateFairValue_0F9BC2D6ED138B660036224C2ABCAE32" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_AA055FFD6B426899E37C224C2ABCE94F" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_D187BFCE52CD217281C5224C2ABC929F" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsForfeituresWeightedAverageGrantDateFairValue_AA055FFD6B426899E37C224C2ABCE94F" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_569CC429BA7CB4B7A8EA224C2ABC7B7E" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsNonvestedWeightedAverageGrantDateFairValueRollForward_D187BFCE52CD217281C5224C2ABC929F" xlink:to="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardEquityInstrumentsOtherThanOptionsVestedInPeriodWeightedAverageGrantDateFairValue_569CC429BA7CB4B7A8EA224C2ABC7B7E" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_F0103D88D2191CFA08E4224C2ABD574A" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C485CDD00A6A8C9BE2B9224C2ABC47ED" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognized_F0103D88D2191CFA08E4224C2ABD574A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1" xlink:label="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_8641ED6F59EDE889E751224C2ABD38EB" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C485CDD00A6A8C9BE2B9224C2ABC47ED" xlink:to="loc_us-gaap_EmployeeServiceShareBasedCompensationNonvestedAwardsTotalCompensationCostNotYetRecognizedPeriodForRecognition1_8641ED6F59EDE889E751224C2ABD38EB" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation" xlink:label="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_705CFD017157C4E8A40F224C2ABD4FB2" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C485CDD00A6A8C9BE2B9224C2ABC47ED" xlink:to="loc_us-gaap_SharesPaidForTaxWithholdingForShareBasedCompensation_705CFD017157C4E8A40F224C2ABD4FB2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation" xlink:label="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_B27362C08B205036E408224C2ABD60EA" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_C485CDD00A6A8C9BE2B9224C2ABC47ED" xlink:to="loc_us-gaap_PaymentsRelatedToTaxWithholdingForShareBasedCompensation_B27362C08B205036E408224C2ABD60EA" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/StockholdersEquityShareRepurchaseProgramDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems" xlink:label="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0CFB54C67ACB438B0E2C2C7BEA5D8EA8" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable" xlink:label="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_71BA4216FCA5F116AD1B2C7BEA5DC43B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0CFB54C67ACB438B0E2C2C7BEA5D8EA8" xlink:to="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_71BA4216FCA5F116AD1B2C7BEA5DC43B" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareRepurchaseProgramAxis" xlink:label="loc_us-gaap_ShareRepurchaseProgramAxis_104D48A65507A7AEDED92C7BEA5D5DA8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_ScheduleOfShareBasedCompensationArrangementsByShareBasedPaymentAwardTable_71BA4216FCA5F116AD1B2C7BEA5DC43B" xlink:to="loc_us-gaap_ShareRepurchaseProgramAxis_104D48A65507A7AEDED92C7BEA5D5DA8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_636649201D12400E929D2C7BEA5D08DA_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_104D48A65507A7AEDED92C7BEA5D5DA8" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_636649201D12400E929D2C7BEA5D08DA_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_636649201D12400E929D2C7BEA5D08DA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_104D48A65507A7AEDED92C7BEA5D5DA8" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_636649201D12400E929D2C7BEA5D08DA" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_ShareRepurchaseProgramMember" xlink:label="loc_ftnt_ShareRepurchaseProgramMember_72C863AB0AA78411ACF62C7BEA5D2ACA" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareRepurchaseProgramDomain_636649201D12400E929D2C7BEA5D08DA" xlink:to="loc_ftnt_ShareRepurchaseProgramMember_72C863AB0AA78411ACF62C7BEA5D2ACA" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockRepurchaseProgramAuthorizedAmount1" xlink:label="loc_us-gaap_StockRepurchaseProgramAuthorizedAmount1_A71948FE209D32501A0E2C7BEA5DEBE9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0CFB54C67ACB438B0E2C2C7BEA5D8EA8" xlink:to="loc_us-gaap_StockRepurchaseProgramAuthorizedAmount1_A71948FE209D32501A0E2C7BEA5DEBE9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodShares" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_30EB4E10149EDD6009A12C7BEA5DFEA0" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0CFB54C67ACB438B0E2C2C7BEA5D8EA8" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodShares_30EB4E10149EDD6009A12C7BEA5DFEA0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_TreasuryStockAcquiredAverageCostPerShare" xlink:label="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_C565B0F9FBC166066D562C7BEA5D0CFF" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0CFB54C67ACB438B0E2C2C7BEA5D8EA8" xlink:to="loc_us-gaap_TreasuryStockAcquiredAverageCostPerShare_C565B0F9FBC166066D562C7BEA5D0CFF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockRepurchasedAndRetiredDuringPeriodValue" xlink:label="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_38D40D777996F6358C812C7BEA5ECCBF" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0CFB54C67ACB438B0E2C2C7BEA5D8EA8" xlink:to="loc_us-gaap_StockRepurchasedAndRetiredDuringPeriodValue_38D40D777996F6358C812C7BEA5ECCBF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1" xlink:label="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_DE65562AC5CE7C273B7C2C7BEA5E6E7E" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareBasedCompensationArrangementByShareBasedPaymentAwardLineItems_0CFB54C67ACB438B0E2C2C7BEA5D8EA8" xlink:to="loc_us-gaap_StockRepurchaseProgramRemainingAuthorizedRepurchaseAmount1_DE65562AC5CE7C273B7C2C7BEA5E6E7E" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/StockholdersEquityStockBasedCompensationPlanDetails" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/StockholdersEquityTables" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/SubsequentEvent" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/SubsequentEventDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SubsequentEventLineItems" xlink:label="loc_us-gaap_SubsequentEventLineItems_4480FFCE1A925AF6C3783FD50A970D10" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SubsequentEventTable" xlink:label="loc_us-gaap_SubsequentEventTable_EFCFA74C97348314A47F3FD50A95661E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_SubsequentEventLineItems_4480FFCE1A925AF6C3783FD50A970D10" xlink:to="loc_us-gaap_SubsequentEventTable_EFCFA74C97348314A47F3FD50A95661E" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareRepurchaseProgramAxis" xlink:label="loc_us-gaap_ShareRepurchaseProgramAxis_526D0E01218BC697280B3FD50A969CA9" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_EFCFA74C97348314A47F3FD50A95661E" xlink:to="loc_us-gaap_ShareRepurchaseProgramAxis_526D0E01218BC697280B3FD50A969CA9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_23A69D6A628CD695C67F3FD50A96C468_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_526D0E01218BC697280B3FD50A969CA9" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_23A69D6A628CD695C67F3FD50A96C468_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ShareRepurchaseProgramDomain" xlink:label="loc_us-gaap_ShareRepurchaseProgramDomain_23A69D6A628CD695C67F3FD50A96C468" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_ShareRepurchaseProgramAxis_526D0E01218BC697280B3FD50A969CA9" xlink:to="loc_us-gaap_ShareRepurchaseProgramDomain_23A69D6A628CD695C67F3FD50A96C468" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_ShareRepurchaseProgramMember" xlink:label="loc_ftnt_ShareRepurchaseProgramMember_B0ADCACF61C3D8776F533FD50A970078" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ShareRepurchaseProgramDomain_23A69D6A628CD695C67F3FD50A96C468" xlink:to="loc_ftnt_ShareRepurchaseProgramMember_B0ADCACF61C3D8776F533FD50A970078" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SubsequentEventTypeAxis" xlink:label="loc_us-gaap_SubsequentEventTypeAxis_76DDE417EACBD2F78B593FD50A975B8B" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_SubsequentEventTable_EFCFA74C97348314A47F3FD50A95661E" xlink:to="loc_us-gaap_SubsequentEventTypeAxis_76DDE417EACBD2F78B593FD50A975B8B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_A49FC81DB600A4FFACEF3FD50A97AED3_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_76DDE417EACBD2F78B593FD50A975B8B" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_A49FC81DB600A4FFACEF3FD50A97AED3_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SubsequentEventTypeDomain" xlink:label="loc_us-gaap_SubsequentEventTypeDomain_A49FC81DB600A4FFACEF3FD50A97AED3" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_SubsequentEventTypeAxis_76DDE417EACBD2F78B593FD50A975B8B" xlink:to="loc_us-gaap_SubsequentEventTypeDomain_A49FC81DB600A4FFACEF3FD50A97AED3" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_SubsequentEventMember" xlink:label="loc_us-gaap_SubsequentEventMember_9435792EC898382489173FD50A97485E" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventTypeDomain_A49FC81DB600A4FFACEF3FD50A97AED3" xlink:to="loc_us-gaap_SubsequentEventMember_9435792EC898382489173FD50A97485E" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_StockRepurchaseProgramAdditionalAuthorizedAmount" xlink:label="loc_ftnt_StockRepurchaseProgramAdditionalAuthorizedAmount_964730D64AD87271470E3FD50A97109B" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_4480FFCE1A925AF6C3783FD50A970D10" xlink:to="loc_ftnt_StockRepurchaseProgramAdditionalAuthorizedAmount_964730D64AD87271470E3FD50A97109B" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StockRepurchaseProgramAuthorizedAmount1" xlink:label="loc_us-gaap_StockRepurchaseProgramAuthorizedAmount1_B6DC9A5CC22B6212F7B03FD50A972999" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_SubsequentEventLineItems_4480FFCE1A925AF6C3783FD50A970D10" xlink:to="loc_us-gaap_StockRepurchaseProgramAuthorizedAmount1_B6DC9A5CC22B6212F7B03FD50A972999" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/SummaryOfSignificantAccountingPolicies" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/SummaryOfSignificantAccountingPoliciesImpactOfAdoptionOfNewAccountingStandardDetails" xlink:type="extended">
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems" xlink:label="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_350DB034E5730F3E4C8980A2AD78DA94" xlink:type="locator" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable" xlink:label="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_8808897A64003A34C20880A2AD76C26A" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/all" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_350DB034E5730F3E4C8980A2AD78DA94" xlink:to="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_8808897A64003A34C20880A2AD76C26A" xlink:type="arc" xbrldt:closed="true" xbrldt:contextElement="segment" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeAxis" xlink:label="loc_us-gaap_RangeAxis_623E429FEDD3516C8CD380A2AD770CE5" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_8808897A64003A34C20880A2AD76C26A" xlink:to="loc_us-gaap_RangeAxis_623E429FEDD3516C8CD380A2AD770CE5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_ABDF3334A19755ED786780A2AD770F01_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_RangeAxis_623E429FEDD3516C8CD380A2AD770CE5" xlink:to="loc_us-gaap_RangeMember_ABDF3334A19755ED786780A2AD770F01_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RangeMember" xlink:label="loc_us-gaap_RangeMember_ABDF3334A19755ED786780A2AD770F01" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_RangeAxis_623E429FEDD3516C8CD380A2AD770CE5" xlink:to="loc_us-gaap_RangeMember_ABDF3334A19755ED786780A2AD770F01" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MinimumMember" xlink:label="loc_us-gaap_MinimumMember_A8F51D655A793AC9208280A2AD77A251" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_ABDF3334A19755ED786780A2AD770F01" xlink:to="loc_us-gaap_MinimumMember_A8F51D655A793AC9208280A2AD77A251" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_MaximumMember" xlink:label="loc_us-gaap_MaximumMember_4F3C62932E0BED55013380A2AD779DD9" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_RangeMember_ABDF3334A19755ED786780A2AD770F01" xlink:to="loc_us-gaap_MaximumMember_4F3C62932E0BED55013380A2AD779DD9" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementEquityComponentsAxis" xlink:label="loc_us-gaap_StatementEquityComponentsAxis_8AC65E2912E0B9C5969180A2AD778837" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_8808897A64003A34C20880A2AD76C26A" xlink:to="loc_us-gaap_StatementEquityComponentsAxis_8AC65E2912E0B9C5969180A2AD778837" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_FD245035CAD1E2A00C0680A2AD778792_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_8AC65E2912E0B9C5969180A2AD778837" xlink:to="loc_us-gaap_EquityComponentDomain_FD245035CAD1E2A00C0680A2AD778792_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EquityComponentDomain" xlink:label="loc_us-gaap_EquityComponentDomain_FD245035CAD1E2A00C0680A2AD778792" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementEquityComponentsAxis_8AC65E2912E0B9C5969180A2AD778837" xlink:to="loc_us-gaap_EquityComponentDomain_FD245035CAD1E2A00C0680A2AD778792" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RetainedEarningsMember" xlink:label="loc_us-gaap_RetainedEarningsMember_0CFEDC676AAC647A514980A2AD7740D8" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_EquityComponentDomain_FD245035CAD1E2A00C0680A2AD778792" xlink:to="loc_us-gaap_RetainedEarningsMember_0CFEDC676AAC647A514980A2AD7740D8" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NewAccountingPronouncementEarlyAdoptionAxis" xlink:label="loc_us-gaap_NewAccountingPronouncementEarlyAdoptionAxis_DC8D7E102968C200AE2880A2AD771128" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_8808897A64003A34C20880A2AD76C26A" xlink:to="loc_us-gaap_NewAccountingPronouncementEarlyAdoptionAxis_DC8D7E102968C200AE2880A2AD771128" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NewAccountingPrinciplesEarlyAdoptionMember" xlink:label="loc_us-gaap_NewAccountingPrinciplesEarlyAdoptionMember_394F188E8B0908C4921F80A2AD775D6F_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_NewAccountingPronouncementEarlyAdoptionAxis_DC8D7E102968C200AE2880A2AD771128" xlink:to="loc_us-gaap_NewAccountingPrinciplesEarlyAdoptionMember_394F188E8B0908C4921F80A2AD775D6F_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NewAccountingPrinciplesEarlyAdoptionMember" xlink:label="loc_us-gaap_NewAccountingPrinciplesEarlyAdoptionMember_394F188E8B0908C4921F80A2AD775D6F" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_NewAccountingPronouncementEarlyAdoptionAxis_DC8D7E102968C200AE2880A2AD771128" xlink:to="loc_us-gaap_NewAccountingPrinciplesEarlyAdoptionMember_394F188E8B0908C4921F80A2AD775D6F" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_AccountingStandardsUpdate201609Member" xlink:label="loc_ftnt_AccountingStandardsUpdate201609Member_55981C13B942F99017C780A2AD770813" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPrinciplesEarlyAdoptionMember_394F188E8B0908C4921F80A2AD775D6F" xlink:to="loc_ftnt_AccountingStandardsUpdate201609Member_55981C13B942F99017C780A2AD770813" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_StatementScenarioAxis" xlink:label="loc_us-gaap_StatementScenarioAxis_29D07B6DE70078ED436380A2AD7713E2" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/hypercube-dimension" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleTable_8808897A64003A34C20880A2AD76C26A" xlink:to="loc_us-gaap_StatementScenarioAxis_29D07B6DE70078ED436380A2AD7713E2" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_93188CC37849A4230E3F80A2AD779F78_default" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-default" xlink:from="loc_us-gaap_StatementScenarioAxis_29D07B6DE70078ED436380A2AD7713E2" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_93188CC37849A4230E3F80A2AD779F78_default" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioUnspecifiedDomain" xlink:label="loc_us-gaap_ScenarioUnspecifiedDomain_93188CC37849A4230E3F80A2AD779F78" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/dimension-domain" xlink:from="loc_us-gaap_StatementScenarioAxis_29D07B6DE70078ED436380A2AD7713E2" xlink:to="loc_us-gaap_ScenarioUnspecifiedDomain_93188CC37849A4230E3F80A2AD779F78" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioAdjustmentMember" xlink:label="loc_us-gaap_ScenarioAdjustmentMember_CEF390620C27BB0F662080A2AD778930" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScenarioUnspecifiedDomain_93188CC37849A4230E3F80A2AD779F78" xlink:to="loc_us-gaap_ScenarioAdjustmentMember_CEF390620C27BB0F662080A2AD778930" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_ScenarioPreviouslyReportedMember" xlink:label="loc_us-gaap_ScenarioPreviouslyReportedMember_634F7F1E58B502D28EA380A2AD78496D" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_ScenarioUnspecifiedDomain_93188CC37849A4230E3F80A2AD779F78" xlink:to="loc_us-gaap_ScenarioPreviouslyReportedMember_634F7F1E58B502D28EA380A2AD78496D" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption" xlink:label="loc_us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption_FF7AF9BE5E0EAC62561080A2AD78AF15" xlink:type="locator" />
    <definitionArc order="1" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_350DB034E5730F3E4C8980A2AD78DA94" xlink:to="loc_us-gaap_CumulativeEffectOfNewAccountingPrincipleInPeriodOfAdoption_FF7AF9BE5E0EAC62561080A2AD78AF15" xlink:type="arc" />
    <loc xlink:href="ftnt-20160930.xsd#ftnt_EffectiveIncomeTaxRateReconciliationSharebasedCompensationExcessTaxBenefitAmount" xlink:label="loc_ftnt_EffectiveIncomeTaxRateReconciliationSharebasedCompensationExcessTaxBenefitAmount_DE1DDF91BE6E3A16EEEF80A2AD78630A" xlink:type="locator" />
    <definitionArc order="2" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_350DB034E5730F3E4C8980A2AD78DA94" xlink:to="loc_ftnt_EffectiveIncomeTaxRateReconciliationSharebasedCompensationExcessTaxBenefitAmount_DE1DDF91BE6E3A16EEEF80A2AD78630A" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AllocatedShareBasedCompensationExpense" xlink:label="loc_us-gaap_AllocatedShareBasedCompensationExpense_FC0AE7CB8042BCC1BD8580A2AD78C748" xlink:type="locator" />
    <definitionArc order="3" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_350DB034E5730F3E4C8980A2AD78DA94" xlink:to="loc_us-gaap_AllocatedShareBasedCompensationExpense_FC0AE7CB8042BCC1BD8580A2AD78C748" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_IncomeTaxExpenseBenefit" xlink:label="loc_us-gaap_IncomeTaxExpenseBenefit_5A91AE0DDEEB24E0269A80A2AD78C688" xlink:type="locator" />
    <definitionArc order="4" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_350DB034E5730F3E4C8980A2AD78DA94" xlink:to="loc_us-gaap_IncomeTaxExpenseBenefit_5A91AE0DDEEB24E0269A80A2AD78C688" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_NetIncomeLoss" xlink:label="loc_us-gaap_NetIncomeLoss_3C652787E5C110E55FC480A2AD787CF0" xlink:type="locator" />
    <definitionArc order="5" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_350DB034E5730F3E4C8980A2AD78DA94" xlink:to="loc_us-gaap_NetIncomeLoss_3C652787E5C110E55FC480A2AD787CF0" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EarningsPerShareBasic" xlink:label="loc_us-gaap_EarningsPerShareBasic_33CCAF69452208257E4C80A2AD78DBC5" xlink:type="locator" />
    <definitionArc order="6" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_350DB034E5730F3E4C8980A2AD78DA94" xlink:to="loc_us-gaap_EarningsPerShareBasic_33CCAF69452208257E4C80A2AD78DBC5" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_EarningsPerShareDiluted" xlink:label="loc_us-gaap_EarningsPerShareDiluted_8386B337E249D5B4A71680A2AD7865D4" xlink:type="locator" />
    <definitionArc order="7" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_350DB034E5730F3E4C8980A2AD78DA94" xlink:to="loc_us-gaap_EarningsPerShareDiluted_8386B337E249D5B4A71680A2AD7865D4" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding" xlink:label="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_15E3D6DBE547D5196BF180A2AD78BC67" xlink:type="locator" />
    <definitionArc order="8" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_350DB034E5730F3E4C8980A2AD78DA94" xlink:to="loc_us-gaap_WeightedAverageNumberOfDilutedSharesOutstanding_15E3D6DBE547D5196BF180A2AD78BC67" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_DeferredTaxAssetsLiabilitiesNetNoncurrent" xlink:label="loc_us-gaap_DeferredTaxAssetsLiabilitiesNetNoncurrent_C7E0CF1594FFEECC278980A2AD7844EF" xlink:type="locator" />
    <definitionArc order="9" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_350DB034E5730F3E4C8980A2AD78DA94" xlink:to="loc_us-gaap_DeferredTaxAssetsLiabilitiesNetNoncurrent_C7E0CF1594FFEECC278980A2AD7844EF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_AdditionalPaidInCapitalCommonStock" xlink:label="loc_us-gaap_AdditionalPaidInCapitalCommonStock_CB693C0E818E9CF9F6BC80A2AD78E7AF" xlink:type="locator" />
    <definitionArc order="10" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_350DB034E5730F3E4C8980A2AD78DA94" xlink:to="loc_us-gaap_AdditionalPaidInCapitalCommonStock_CB693C0E818E9CF9F6BC80A2AD78E7AF" xlink:type="arc" />
    <loc xlink:href="http://xbrl.fasb.org/us-gaap/2015/elts/us-gaap-2015-01-31.xsd#us-gaap_RetainedEarningsAccumulatedDeficit" xlink:label="loc_us-gaap_RetainedEarningsAccumulatedDeficit_20027926AD48F94ED9AF80A2AD79472A" xlink:type="locator" />
    <definitionArc order="11" xlink:arcrole="http://xbrl.org/int/dim/arcrole/domain-member" xlink:from="loc_us-gaap_NewAccountingPronouncementsOrChangeInAccountingPrincipleLineItems_350DB034E5730F3E4C8980A2AD78DA94" xlink:to="loc_us-gaap_RetainedEarningsAccumulatedDeficit_20027926AD48F94ED9AF80A2AD79472A" xlink:type="arc" />
  </definitionLink>
  <definitionLink xlink:role="http://www.fortinet.com/role/SummaryOfSignificantAccountingPoliciesPolicies" xlink:type="extended" />
  <definitionLink xlink:role="http://www.fortinet.com/role/SummaryOfSignificantAccountingPoliciesTables" xlink:type="extended" />
</linkbase>
