-----BEGIN PRIVACY-ENHANCED MESSAGE-----
Proc-Type: 2001,MIC-CLEAR
Originator-Name: webmaster@www.sec.gov
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<SEC-DOCUMENT>0001222572-04-000071.txt : 20041216
<SEC-HEADER>0001222572-04-000071.hdr.sgml : 20041216
<ACCEPTANCE-DATETIME>20041216105244
ACCESSION NUMBER:		0001222572-04-000071
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20041216
FILED AS OF DATE:		20041216
DATE AS OF CHANGE:		20041216

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			PEGASYSTEMS INC
		CENTRAL INDEX KEY:			0001013857
		STANDARD INDUSTRIAL CLASSIFICATION:	SERVICES-COMPUTER PROCESSING & DATA PREPARATION [7374]
		IRS NUMBER:				042787865
		STATE OF INCORPORATION:			MA
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		101 MAIN ST
		CITY:			CAMBRIDGE
		STATE:			MA
		ZIP:			02142-1590
		BUSINESS PHONE:		6173749600

	MAIL ADDRESS:	
		STREET 1:		101 MAIN ST
		CITY:			CAMBRIDGE
		STATE:			MA
		ZIP:			02142-1590

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			DARBELOFF ALEXANDER V
		CENTRAL INDEX KEY:			0001222572

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-11859
		FILM NUMBER:		041206732

	MAIL ADDRESS:	
		STREET 1:		C/O PEGASYSTEMS INC
		STREET 2:		101 MAIN STREET
		CITY:			CAMBRIDGE
		STATE:			MA
		ZIP:			02142
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>primary_doc.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0202</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2004-12-16</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001013857</issuerCik>
        <issuerName>PEGASYSTEMS INC</issuerName>
        <issuerTradingSymbol>pega</issuerTradingSymbol>
    </issuer>

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            <rptOwnerName>DARBELOFF ALEXANDER V</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>C/O PEGASYSTEMS INC</rptOwnerStreet1>
            <rptOwnerStreet2>101 MAIN STREET</rptOwnerStreet2>
            <rptOwnerCity>CAMBRIDGE</rptOwnerCity>
            <rptOwnerState>MA</rptOwnerState>
            <rptOwnerZipCode>02142</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Stock</value>
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            <transactionDate>
                <value>2004-12-14</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
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            <transactionTimeliness>
                <value></value>
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            <transactionAmounts>
                <transactionShares>
                    <value>300</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>7.5767</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
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                <transactionPricePerShare>
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            <transactionTimeliness>
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            <transactionAmounts>
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                <transactionAcquiredDisposedCode>
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            <transactionTimeliness>
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                <transactionShares>
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                <transactionPricePerShare>
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                <transactionAcquiredDisposedCode>
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                <transactionShares>
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                <transactionPricePerShare>
                    <value>7.52</value>
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                <value></value>
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                <transactionShares>
                    <value>100</value>
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                <transactionPricePerShare>
                    <value>7.53</value>
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                <transactionAcquiredDisposedCode>
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                </transactionAcquiredDisposedCode>
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                <sharesOwnedFollowingTransaction>
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                </directOrIndirectOwnership>
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                <transactionCode>S</transactionCode>
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                <value></value>
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                <transactionShares>
                    <value>600</value>
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                    <value>7.54</value>
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                </directOrIndirectOwnership>
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        <nonDerivativeTransaction>
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                <value>Common Stock</value>
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                <value></value>
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                <transactionShares>
                    <value>100</value>
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                <transactionPricePerShare>
                    <value>7.56</value>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
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    <remarks>4,500 shares of common stock have been sold pursuant to the requirements of Rule 10b5-1 Sales Plan under the Securities Exchange Act of 1934, as amended.</remarks>

    <ownerSignature>
        <signatureName>Shawn Hoyt, Esq., as Attorney-in-Fact for Alexander V. d'Arbeloff</signatureName>
        <signatureDate>2004-12-16</signatureDate>
    </ownerSignature>
</ownershipDocument>

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</TEXT>
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