<ownershipDocument>
    <schemaVersion>X0101</schemaVersion>

    <documentType>3</documentType>
<periodOfReport>2003-07-01</periodOfReport>
    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0000031462</issuerCik>
        <issuerName>ECOLAB INC</issuerName>
        <issuerTradingSymbol>ECL</issuerTradingSymbol>
</issuer>
<reportingOwner><reportingOwnerId>
            <rptOwnerCik>0001229313</rptOwnerCik>
            <rptOwnerName>NEWLIN STEPHEN D</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>  </rptOwnerStreet1>
            <rptOwnerStreet2>  </rptOwnerStreet2>
            <rptOwnerCity>  </rptOwnerCity>
            <rptOwnerState>  </rptOwnerState>
            <rptOwnerZipCode>  </rptOwnerZipCode>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>0</isDirector>
<isOfficer>1</isOfficer>
<isTenPercentOwner>0</isTenPercentOwner>
<isOther>0</isOther>
<officerTitle>President - Industrial Sector</officerTitle>
<otherText></otherText>
</reportingOwnerRelationship>
</reportingOwner>
<nonDerivativeSecurity><securityTitle><value>None</value>
</securityTitle>
<transactionAmounts><transactionShares><value>0</value>
</transactionShares>
</transactionAmounts>
<ownershipNature><directOrIndirectOwnership><value>D</value>
</directOrIndirectOwnership>
</ownershipNature>
</nonDerivativeSecurity>
<footnotes></footnotes>
<ownerSignature><signatureName>/s/ Stephen D. Newlin</signatureName>
<signatureDate>2003-07-08</signatureDate>
</ownerSignature>
</ownershipDocument>

