-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
Originator-Key-Asymmetric:
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 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 F7mI7Px1E+kRNga7GDLaEbUK2yS4NNCdihQ73QbSovnCvAbx3766f/JSNaauezkk
 QQS2BUC6fxtoQQvIIblgTA==

<SEC-DOCUMENT>0000077360-05-000006.txt : 20050103
<SEC-HEADER>0000077360-05-000006.hdr.sgml : 20041231
<ACCEPTANCE-DATETIME>20050103154621
ACCESSION NUMBER:		0000077360-05-000006
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20050102
FILED AS OF DATE:		20050103
DATE AS OF CHANGE:		20050103

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			SCHROCK MICHAEL V
		CENTRAL INDEX KEY:			0001189053

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-11625
		FILM NUMBER:		05502516

	MAIL ADDRESS:	
		STREET 1:		5500 WAYZATA BLVD.
		STREET 2:		SUITE 800
		CITY:			GOLDEN VALLEY
		STATE:			MN
		ZIP:			55416

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			PENTAIR INC
		CENTRAL INDEX KEY:			0000077360
		STANDARD INDUSTRIAL CLASSIFICATION:	SPECIAL INDUSTRY MACHINERY (NO METALWORKING MACHINERY) [3550]
		IRS NUMBER:				410907434
		STATE OF INCORPORATION:			MN
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		5500 WAYZATA BLVD.
		STREET 2:		SUITE 800
		CITY:			GOLDEN VALLEY
		STATE:			MN
		ZIP:			55416
		BUSINESS PHONE:		763-545-1730

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	PENTAIR INDUSTRIES INC
		DATE OF NAME CHANGE:	19790327
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>c0001189053d20050103f4f.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2005-01-02</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0000077360</issuerCik>
        <issuerName>PENTAIR INC</issuerName>
        <issuerTradingSymbol>PNR</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001189053</rptOwnerCik>
            <rptOwnerName>SCHROCK MICHAEL V</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>5500 WAYZATA BLVD.</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 800</rptOwnerStreet2>
            <rptOwnerCity>GOLDEN VALLEY</rptOwnerCity>
            <rptOwnerState>MN</rptOwnerState>
            <rptOwnerZipCode>55416-1259</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>0</isDirector>
            <isOfficer>1</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle>President, COO Enclosures</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable></nonDerivativeTable>

    <derivativeTable>
        <derivativeTransaction>
            <securityTitle>
                <value>Incentive Bonus Award (cash-only right)</value>
            </securityTitle>
            <conversionOrExercisePrice>
                <value>0</value>
            </conversionOrExercisePrice>
            <transactionDate>
                <value>2005-01-02</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>J</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F1"/>
            </transactionCoding>
            <transactionAmounts>
                <transactionShares>
                    <value>288000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>0</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <exerciseDate>
                <value>2005-01-02</value>
            </exerciseDate>
            <expirationDate>
                <value>2005-01-02</value>
            </expirationDate>
            <underlyingSecurity>
                <underlyingSecurityTitle>
                    <value>Cash-only Right</value>
                </underlyingSecurityTitle>
                <underlyingSecurityShares>
                    <value>288000</value>
                </underlyingSecurityShares>
            </underlyingSecurity>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>0</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </derivativeTransaction>
    </derivativeTable>

    <footnotes>
        <footnote id="F1">Payment of vested incentive bonus award based on a fixed value as determined by the Compensation Committee.</footnote>
        <footnote id="F2">On May 17, 2004, Pentair declared a 2-for-1 stock split in the form of a 100 percent stock dividend payable on June 8, 2004, to shareholders of record as of June 1, 2004.  This previously reported derivative security has been adjusted to reflect the stock split.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Louis L. Ainsworth, Attorney-In-Fact</signatureName>
        <signatureDate>2005-01-03</signatureDate>
    </ownerSignature>
</ownershipDocument>

</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
-----END PRIVACY-ENHANCED MESSAGE-----