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Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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MIC-Info: RSA-MD5,RSA,
 HyiIh/DPbVNcOpsVKfntt3QZsBz+6GzlJc/sk1IjLB3SNe3QkpG22WMgzQhGe9Wt
 03T7c545rcLX478Eyr2Rrw==

<SEC-DOCUMENT>0000077360-08-000128.txt : 20081030
<SEC-HEADER>0000077360-08-000128.hdr.sgml : 20081030
<ACCEPTANCE-DATETIME>20081030152625
ACCESSION NUMBER:		0000077360-08-000128
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20081028
FILED AS OF DATE:		20081030
DATE AS OF CHANGE:		20081030

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

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

	FORMER COMPANY:	
		FORMER CONFORMED NAME:	PENTAIR INDUSTRIES INC
		DATE OF NAME CHANGE:	19790327

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			JONES DAVID A /WI
		CENTRAL INDEX KEY:			0000905631
		STATE OF INCORPORATION:			KY
		FISCAL YEAR END:			1231

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	000-04689
		FILM NUMBER:		081150881

	BUSINESS ADDRESS:	
		BUSINESS PHONE:		6082753340

	MAIL ADDRESS:	
		STREET 1:		C/O RAYOVAC CORP
		STREET 2:		601 RAYOVAC DR
		CITY:			MADISON
		STATE:			WI
		ZIP:			53711-2497
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>doc.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2008-10-28</periodOfReport>

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

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0000905631</rptOwnerCik>
            <rptOwnerName>JONES DAVID A /WI</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>5500 WAYZATA BLVD.</rptOwnerStreet1>
            <rptOwnerStreet2>SUITE 800</rptOwnerStreet2>
            <rptOwnerCity>GOLDEN VALLEY</rptOwnerCity>
            <rptOwnerState>MN</rptOwnerState>
            <rptOwnerZipCode>55416-1261</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
            <officerTitle></officerTitle>
            <otherText></otherText>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
            </securityTitle>
            <transactionDate>
                <value>2008-10-28</value>
            </transactionDate>
            <deemedExecutionDate></deemedExecutionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>P</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>5800</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>24.22</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>A</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>5800</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <derivativeTable></derivativeTable>

    <footnotes>
        <footnote id="F1">The reported price is the weighted average price.  These shares were purchased in multiple transactions at prices ranging from $24.08 to $24.29, inclusive.  The reporting person undertakes to provide to Pentair, Inc., any security holder of Pentair, Inc., or the staff of the Securities and Exchange Commission, upon request, full information regarding the number of shares purchased at each separate price.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Louis L. Ainsworth, Attorney-in-Fact</signatureName>
        <signatureDate>2008-10-30</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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