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

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2005-08-23</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001327463</rptOwnerCik>
            <rptOwnerName>Olson David W</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>21650 OXNARD STREET</rptOwnerStreet1>
            <rptOwnerStreet2></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>SVP, Corporate Communications</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2005-08-23</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>1788</value>
                    <footnoteId id="F1"/>
                </transactionShares>
                <transactionPricePerShare>
                    <value>42.31</value>
                    <footnoteId id="F2"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4937</value>
                    <footnoteId id="F3"/>
                </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 a 5,000 share restricted stock grant made to the Reporting Person on August 12, 2002.</footnote>
        <footnote id="F2">The price used to calculate the number of shares forfeited to the Issuer by the Reporting Person was based on the closing price of the Issuer's common stock on August 12, 2005 (the vesting date of the August 12, 2002 restricted stock grant).  The Reporting Person notified the Issuer on August 23, 2005 of his intent to forfeit shares to satisfy payment of the tax liability on the vested shares.</footnote>
        <footnote id="F3">Includes 225 shares in which the Reporting Person has a beneficial ownership interest under the profit sharing component of the Issuer's 401(k) Savings Plan.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>/s/ David W. Olson</signatureName>
        <signatureDate>2005-08-23</signatureDate>
    </ownerSignature>
</ownershipDocument>
