<ownershipDocument>
    <schemaVersion>X0201</schemaVersion>

    <documentType>3/A</documentType>
<periodOfReport>2003-12-23</periodOfReport>
<dateOfOriginalSubmission>2003-12-23</dateOfOriginalSubmission>
    <noSecuritiesOwned>0</noSecuritiesOwned>

    <issuer>
        <issuerCik>0000073088</issuerCik>
        <issuerName>NORTHWESTERN CORP</issuerName>
        <issuerTradingSymbol>NTHWQ</issuerTradingSymbol>
</issuer>
<reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001273889</rptOwnerCik>
            <rptOwnerName>SCHRUM ROGER P</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>125 S. DAKOTA AVE.</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>SIOUX FALLS</rptOwnerCity>
            <rptOwnerState>SD</rptOwnerState>
            <rptOwnerZipCode>57104</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
            <reportingOwnerRelationship>
<isDirector>0</isDirector>
<isOfficer>1</isOfficer>
<isTenPercentOwner>0</isTenPercentOwner>
<isOther>0</isOther>
<officerTitle>VP Human Resources &amp; Communica</officerTitle>
<otherText></otherText>
</reportingOwnerRelationship>
</reportingOwner>
<nonDerivativeTable></nonDerivativeTable>
<derivativeTable><derivativeHolding><securityTitle><value>INCENTIVE STOCK OPTION GRANT</value>
</securityTitle>
<conversionOrExercisePrice><value>20.5000</value>
</conversionOrExercisePrice>
<exerciseDate><footnoteId id="F1"></footnoteId>
</exerciseDate>
<expirationDate><footnoteId id="F2"></footnoteId>
</expirationDate>
<underlyingSecurity><underlyingSecurityTitle><value>COMMON STOCK</value>
</underlyingSecurityTitle>
<underlyingSecurityShares><value>3500</value>
</underlyingSecurityShares>
</underlyingSecurity>
<ownershipNature><directOrIndirectOwnership><value>D</value>
</directOrIndirectOwnership>
</ownershipNature>
</derivativeHolding>
</derivativeTable>
<footnotes><footnote id="F1">INCENTIVE STOCK OPTION GRANTED ON MAY 1, 2002, VEST IN FOUR EQUAL ANNUAL INSTALLMENTS BEGINNING ON THE 1SGT ANNIVERSARY OF THE AWARD.</footnote>
<footnote id="F2">THE OPTION EXPIRES TEN YEARS FROM THE DATE OF ISSUE.</footnote>
</footnotes>
<ownerSignature><signatureName>Roger P. Schrum</signatureName>
<signatureDate>2003-12-23</signatureDate>
</ownerSignature>
</ownershipDocument>

