<?xml version="1.0"?>
<ownershipDocument>
    <schemaVersion>X0101</schemaVersion>

    <documentType>4</documentType>
<periodOfReport>2003-05-22</periodOfReport>
<notSubjectToSection16>0</notSubjectToSection16>
    <issuer>
        <issuerCik>0001080099</issuerCik>
        <issuerName>INFORMATICA CORP</issuerName>
        <issuerTradingSymbol>INFA</issuerTradingSymbol>
</issuer>
<reportingOwner>
<reportingOwnerId>
            <rptOwnerCik>0001197652</rptOwnerCik>
            <rptOwnerName>SEAWELL A BROOKE</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>  </rptOwnerStreet1>
            <rptOwnerStreet2>  </rptOwnerStreet2>
            <rptOwnerCity>  </rptOwnerCity>
            <rptOwnerState>  </rptOwnerState>
            <rptOwnerZipCode>  </rptOwnerZipCode>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>1</isDirector>
<isOfficer>0</isOfficer>
<isTenPercentOwner>0</isTenPercentOwner>
<isOther>0</isOther>
<officerTitle></officerTitle>
<otherText></otherText>
</reportingOwnerRelationship>
</reportingOwner>
<derivativeSecurity>
<securityTitle>
<value>Non-Qualified Stock Option (right to buy)</value>
</securityTitle>
<conversionOrExercisePrice>
<value>6.91</value>
</conversionOrExercisePrice>
<transactionDate>
<value>2003-05-22</value>
</transactionDate>
<transactionCoding>
<transactionFormType>4</transactionFormType>
<transactionCode>A</transactionCode>
<equitySwapInvolved>0</equitySwapInvolved>
</transactionCoding>
<transactionTimeliness>
<value></value>
</transactionTimeliness>
<transactionAmounts>
<transactionShares>
<value>20000</value>
</transactionShares>
<transactionValue>
<value>0</value>
</transactionValue>
<transactionAcquiredDisposedCode>
<value>A</value>
<footnoteId id="F1"/>
</transactionAcquiredDisposedCode>
</transactionAmounts>
<exerciseDate>
<value>2004-05-22</value>
<footnoteId id="F1"/>
</exerciseDate>
<expirationDate>
<value>2008-05-22</value>
</expirationDate>
<underlyingSecurity>
<underlyingSecurityTitle>
<value>Common Stock</value>
</underlyingSecurityTitle>
<underlyingSecurityShares>
<value>20000</value>
</underlyingSecurityShares>
</underlyingSecurity>
<postTransactionAmounts>
<sharesOwnedFollowingTransaction>
<value>20000</value>
</sharesOwnedFollowingTransaction>
</postTransactionAmounts>
<ownershipNature>
<directOrIndirectOwnership>
<value>D</value>
</directOrIndirectOwnership>
<natureOfOwnership>
<value></value>
</natureOfOwnership>
</ownershipNature>
</derivativeSecurity>
<footnotes>
<footnote id="F1">Shares acquired pursuant to the 1999 Nonemployee Director Stock Incentive Plan of Informatica Corporation and shall be fully vested and exercisable on May 22, 2004.</footnote>
</footnotes>
<ownerSignature>
<signatureName>Attorney-in-fact for A. Brooke Seawell: Peter McGoff</signatureName>
<signatureDate>2003-05-27</signatureDate>
</ownerSignature>
</ownershipDocument>

