<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0306</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2014-03-07</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000916085</issuerCik>
        <issuerName>HEALTH NET INC</issuerName>
        <issuerTradingSymbol>HNT</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001450550</rptOwnerCik>
            <rptOwnerName>Tough Steven D</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O HEALTH NET, INC.</rptOwnerStreet1>
            <rptOwnerStreet2>21650 OXNARD STREET</rptOwnerStreet2>
            <rptOwnerCity>WOODLAND HILLS</rptOwnerCity>
            <rptOwnerState>CA</rptOwnerState>
            <rptOwnerZipCode>91367</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>Pres, Govt Progrms</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2014-03-07</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>5011</value>
                    <footnoteId id="F2"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>34.71</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>115669</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">Shares forfeited in lieu of payment of tax liability in connection with the vesting of 33-1/3% of a grant of 10,000 restricted stock units (the &quot;RSU Vest&quot;) and 33-1/3% of a grant of 30,000 performance share units (the &quot;PSU Vest&quot;), each of which was made to the Reporting Person on March 7, 2013, pursuant to the Issuer's 2006 Long-Term Incentive Plan, as amended (the &quot;2006 Plan&quot;).</footnote>
        <footnote id="F2">Represents 1,253 and 3,758 shares forfeited in lieu of payment of tax liability in connection with each of the RSU Vest and PSU Vest, respectively. The number of shares forfeited by the Reporting Person in connection with each of the RSU Vest and PSU Vest was based on the closing price of the Issuer's common stock on March 7, 2014, pursuant to the terms of the 2006 Plan.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ Steven D. Tough</signatureName>
        <signatureDate>2014-03-11</signatureDate>
    </ownerSignature>
</ownershipDocument>
