-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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 TWSM7vrzLADbmYQaionwg5sDW3P6oaM5D3tdezXMm7z1T+B+twIDAQAB
MIC-Info: RSA-MD5,RSA,
 WNSnjMJL1H/vH0Hv81ZOvRp3s6iQSPA6YCANRsEkmw400y4cdMvxCoadaP36BSUG
 IPZpFfEcSxRUQTq3ipeGiQ==

<SEC-DOCUMENT>0000077360-04-000138.txt : 20040817
<SEC-HEADER>0000077360-04-000138.hdr.sgml : 20040817
<ACCEPTANCE-DATETIME>20040817161024
ACCESSION NUMBER:		0000077360-04-000138
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20040813
FILED AS OF DATE:		20040817

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			CATHCART RICHARD J
		CENTRAL INDEX KEY:			0001189039

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-11625
		FILM NUMBER:		04982110

	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>c0001189039d20040817f4f.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2004-08-13</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001189039</rptOwnerCik>
            <rptOwnerName>CATHCART RICHARD J</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 Water Tech.</officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>120210</value>
                    <footnoteId id="F1"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>1524.371</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By Children</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock - ESPP</value>
            </securityTitle>
            <transactionDate>
                <value>2004-08-13</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>5</transactionFormType>
                <transactionCode>A</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
                <footnoteId id="F3"/>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>29.689</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>31.5771</value>
                    <footnoteId id="F4"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>4926.787</value>
                    <footnoteId id="F5"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
        <nonDerivativeHolding>
            <securityTitle>
                <value>Common Stock - ESOP</value>
            </securityTitle>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>3667.8661</value>
                    <footnoteId id="F2"/>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>I</value>
                </directOrIndirectOwnership>
                <natureOfOwnership>
                    <value>By ESOP</value>
                </natureOfOwnership>
            </ownershipNature>
        </nonDerivativeHolding>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">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.  End-of-period holdings have been adjusted accordingly.</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.  In addition, end-of-period holdings include shares acquired under a dividend reinvestment plan in exempt transaction not required to be reported pursuant to Section 16(a).</footnote>
        <footnote id="F3">Purchase of shares under the Pentair, Inc. Employee Stock Purchase Plan in exempt transaction pursuant to Rule 16b-3(c).</footnote>
        <footnote id="F4">Shares acquired under this Plan are purchased over a three-day trading period.  The price shown is the average purchase price over the trading period.</footnote>
        <footnote id="F5">End-of-period holdings include shares acquired under a dividend reinvestment plan in exempt transaction not required to be reported pursuant to Section 16(a).</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>David D. Harrison, Attorney-In-Fact</signatureName>
        <signatureDate>2004-08-17</signatureDate>
    </ownerSignature>
</ownershipDocument>

</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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