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                            [ROPES & GRAY letterhead]





                                February 4, 2000



Forrester Research, Inc.
400 Technology Square
Cambridge, Massachusetts  02139

         Re:   Forrester Research, Inc.

Ladies and Gentlemen:

         This opinion is furnished to you in connection with a registration
statement on Form S-3 (the "Registration Statement"), filed with the Securities
and Exchange Commission under the Securities Act of 1933, as amended, for the
registration of 3,269,450 shares of common stock, $.01 par value (the "Shares"),
of Forrester Research, Inc., a Delaware corporation (the "Company"). The Shares
are to be sold pursuant to an underwriting agreement (the "Underwriting
Agreement") to be entered into among the Company, the stockholders of the
Company named on Schedule II thereto (the "Selling Stockholders") and Goldman,
Sachs & Co., Adams, Harkness & Hill, Inc., Thomas Weisel Partners LLC,
FAC/Equities and William Blair & Company, as representatives of the underwriters
named therein.

         We have acted as counsel for the Company and the Selling Stockholders
in connection with the issuance and sale by the Company, and the sale by the
Selling Stockholders, of the Shares. For purposes of this opinion, we have
examined and relied upon such documents, records, certificates and other
instruments as we have deemed necessary. For purposes of this opinion, we have
assumed that (i) the amendment (the "Amendment") to the Company's Restated
Certificate of Incorporation (the "Restated Certificate") authorized by the
Company's Board of Directors on December 17, 1999 increasing the number of
authorized shares of Common Stock to 125,000,000 shares has been approved by a
majority of the shares of the Common Stock entitled to vote on the Amendment in
accordance with law and the Company's Restated Certificate and (ii) a
certificate of amendment to the Restated Certificate with respect to the
Amendment has been properly filed with the Secretary of State of Delaware in
accordance with the laws of the State of Delaware.
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Forrester Research, Inc.              -2-                       February 4, 2000



         We express no opinion as to the applicability of compliance with or
effect of Federal law or the law of any jurisdiction other than The Commonwealth
of Massachusetts and the corporate laws of the State of Delaware.

         Based on the foregoing, we are of the opinion that the Shares have been
duly authorized and, when the Shares to be sold by the Selling Stockholders that
are issuable upon the exercise of options issued by the Company, have been
issued in accordance with the terms of such options and the Company's stock
option plan and sold pursuant to the Underwriting Agreement, and when the Shares
to be issued and sold by the Company pursuant to the Underwriting Agreement have
been issued and sold and the Company has received the consideration therefor in
accordance with the terms of the Underwriting Agreement, the Shares will be
validly issued, fully paid and non-assessable.

         We hereby consent to your filing this opinion as an exhibit to the
Registration Statement and to the use of our name therein and in the related
prospectus under the caption "Validity of Common Stock".

         It is understood that this opinion is to be used only in connection
with the offer and sale of the Shares while the Registration Statement is in
effect.

                                   Very truly yours,



                                   Ropes & Gray

