-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
Originator-Key-Asymmetric:
 MFgwCgYEVQgBAQICAf8DSgAwRwJAW2sNKK9AVtBzYZmr6aGjlWyK3XmZv3dTINen
 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 Kq5Wcm+LdJRrn2rvH/Pb/n6na29UpeUkhv+JiZ5dn0+rTPMI9rw0SiZzy11I0Pej
 CrhcYo2YVBs5ZsJfObKdlQ==

<SEC-DOCUMENT>0001179110-05-010915.txt : 20050611
<SEC-HEADER>0001179110-05-010915.hdr.sgml : 20050611
<ACCEPTANCE-DATETIME>20050525102733
ACCESSION NUMBER:		0001179110-05-010915
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20050523
FILED AS OF DATE:		20050525
DATE AS OF CHANGE:		20050525

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			NORTHWESTERN CORP
		CENTRAL INDEX KEY:			0000073088
		STANDARD INDUSTRIAL CLASSIFICATION:	ELECTRIC & OTHER SERVICES COMBINED [4931]
		IRS NUMBER:				460172280
		STATE OF INCORPORATION:			DE
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		125 S DAKOTA AVENUE
		STREET 2:		SUITE 1100
		CITY:			SIOUX
		STATE:			SD
		ZIP:			57104
		BUSINESS PHONE:		6059782908

	MAIL ADDRESS:	
		STREET 1:		125 S DAKOTA AVENUE
		STREET 2:		SUITE 1100
		CITY:			SIOUX
		STATE:			SD
		ZIP:			57104

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	NORTHWESTERN PUBLIC SERVICE CO
		DATE OF NAME CHANGE:	19920703

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			Knapp Thomas J
		CENTRAL INDEX KEY:			0001311726

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-10499
		FILM NUMBER:		05855747

	BUSINESS ADDRESS:	
		BUSINESS PHONE:		605-978-2908

	MAIL ADDRESS:	
		STREET 1:		125 S. DAKOTA AVE.
		CITY:			SIOUX FALLS
		STATE:			SD
		ZIP:			57105
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>edgar.xml
<DESCRIPTION>FORM 4 -
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2005-05-23</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000073088</issuerCik>
        <issuerName>NORTHWESTERN CORP</issuerName>
        <issuerTradingSymbol>NWEC</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001311726</rptOwnerCik>
            <rptOwnerName>Knapp Thomas J</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>125 S. DAKOTA AVE.</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>SIOUX FALLS</rptOwnerCity>
            <rptOwnerState>SD</rptOwnerState>
            <rptOwnerZipCode>57105</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>General Counsel</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>COMMON</value>
                <footnoteId id="F1"/>
            </securityTitle>
            <transactionDate>
                <value>2005-05-23</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>F</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>911</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>27.70</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4389</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">SHARES WITHHELD TO PAY LOCAL AND FEDERAL INCOME TAX OBLIGATION FOR RESTRICTED STOCK SHARES VESTING FOLLOWING SIX-MONTH HOLDING PERIOD.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Thomas J. Knapp</signatureName>
        <signatureDate>2005-05-23</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
-----END PRIVACY-ENHANCED MESSAGE-----