-----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,
 ImXZKyh52dAVEx6sp4D0c+Zup/5c0wFW07fS5n6VLdzJLks5Ka/cB1RbTph7xx9I
 PEP7TP5c6SdpqdTflqyaBQ==

<SEC-DOCUMENT>0001254595-09-000024.txt : 20090911
<SEC-HEADER>0001254595-09-000024.hdr.sgml : 20090911
<ACCEPTANCE-DATETIME>20090911163526
ACCESSION NUMBER:		0001254595-09-000024
CONFORMED SUBMISSION TYPE:	4
PUBLIC DOCUMENT COUNT:		1
CONFORMED PERIOD OF REPORT:	20090909
FILED AS OF DATE:		20090911
DATE AS OF CHANGE:		20090911

ISSUER:		

	COMPANY DATA:	
		COMPANY CONFORMED NAME:			FIRST POTOMAC REALTY TRUST
		CENTRAL INDEX KEY:			0001254595
		STANDARD INDUSTRIAL CLASSIFICATION:	REAL ESTATE INVESTMENT TRUSTS [6798]
		IRS NUMBER:				371470730
		STATE OF INCORPORATION:			MD
		FISCAL YEAR END:			1231

	BUSINESS ADDRESS:	
		STREET 1:		7600 WISCONSIN AVENUE
		STREET 2:		11TH FLOOR
		CITY:			BETHESDA
		STATE:			MD
		ZIP:			20814
		BUSINESS PHONE:		3019869200

	MAIL ADDRESS:	
		STREET 1:		7600 WISCONSIN AVENUE
		STREET 2:		11TH FLOOR
		CITY:			BETHESDA
		STATE:			MD
		ZIP:			20814

REPORTING-OWNER:	

	OWNER DATA:	
		COMPANY CONFORMED NAME:			ARNOLD ROBERT H
		CENTRAL INDEX KEY:			0001267222

	FILING VALUES:
		FORM TYPE:		4
		SEC ACT:		1934 Act
		SEC FILE NUMBER:	001-31824
		FILM NUMBER:		091065671

	MAIL ADDRESS:	
		STREET 1:		129 E.69TH ST.
		CITY:			NEW YORK
		STATE:			NY
		ZIP:			10021
</SEC-HEADER>
<DOCUMENT>
<TYPE>4
<SEQUENCE>1
<FILENAME>primary_doc.xml
<DESCRIPTION>PRIMARY DOCUMENT
<TEXT>
<XML>
<?xml version="1.0"?>
<ownershipDocument>

    <schemaVersion>X0303</schemaVersion>

    <documentType>4</documentType>

    <periodOfReport>2009-09-09</periodOfReport>

    <notSubjectToSection16>0</notSubjectToSection16>

    <issuer>
        <issuerCik>0001254595</issuerCik>
        <issuerName>FIRST POTOMAC REALTY TRUST</issuerName>
        <issuerTradingSymbol>FPO</issuerTradingSymbol>
    </issuer>

    <reportingOwner>
        <reportingOwnerId>
            <rptOwnerCik>0001267222</rptOwnerCik>
            <rptOwnerName>ARNOLD ROBERT H</rptOwnerName>
        </reportingOwnerId>
        <reportingOwnerAddress>
            <rptOwnerStreet1>129 E.69TH ST.</rptOwnerStreet1>
            <rptOwnerStreet2></rptOwnerStreet2>
            <rptOwnerCity>NEW YORK</rptOwnerCity>
            <rptOwnerState>NY</rptOwnerState>
            <rptOwnerZipCode>10021</rptOwnerZipCode>
            <rptOwnerStateDescription></rptOwnerStateDescription>
        </reportingOwnerAddress>
        <reportingOwnerRelationship>
            <isDirector>1</isDirector>
            <isOfficer>0</isOfficer>
            <isTenPercentOwner>0</isTenPercentOwner>
            <isOther>0</isOther>
        </reportingOwnerRelationship>
    </reportingOwner>

    <nonDerivativeTable>
        <nonDerivativeTransaction>
            <securityTitle>
                <value>Common Shares</value>
            </securityTitle>
            <transactionDate>
                <value>2009-09-09</value>
            </transactionDate>
            <transactionCoding>
                <transactionFormType>4</transactionFormType>
                <transactionCode>S</transactionCode>
                <equitySwapInvolved>0</equitySwapInvolved>
            </transactionCoding>
            <transactionTimeliness>
                <value></value>
            </transactionTimeliness>
            <transactionAmounts>
                <transactionShares>
                    <value>10000</value>
                </transactionShares>
                <transactionPricePerShare>
                    <value>10.55</value>
                    <footnoteId id="F1"/>
                </transactionPricePerShare>
                <transactionAcquiredDisposedCode>
                    <value>D</value>
                </transactionAcquiredDisposedCode>
            </transactionAmounts>
            <postTransactionAmounts>
                <sharesOwnedFollowingTransaction>
                    <value>12968</value>
                </sharesOwnedFollowingTransaction>
            </postTransactionAmounts>
            <ownershipNature>
                <directOrIndirectOwnership>
                    <value>D</value>
                </directOrIndirectOwnership>
            </ownershipNature>
        </nonDerivativeTransaction>
    </nonDerivativeTable>

    <footnotes>
        <footnote id="F1">Average price.</footnote>
    </footnotes>

    <ownerSignature>
        <signatureName>Joel F. Bonder, Attorney-in-fact for Robert H. Arnold</signatureName>
        <signatureDate>2009-09-11</signatureDate>
    </ownerSignature>
</ownershipDocument>
</XML>
</TEXT>
</DOCUMENT>
</SEC-DOCUMENT>
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