
<PAGE>
 
                                                                     Exhibit 5.1


           [LETTERHEAD OF GOODWIN, PROCTER & HOAR LLP APPEARS HERE]



                                January 23, 1998



Boston Properties, Inc.
8 Arlington Street
Boston, Massachusetts 02116

Gentlemen:

     We have acted as special counsel to Boston Properties, Inc., a Delaware
corporation (the "Company"), in connection with the offer and sale by the
Company of up to 23,000,000 shares of common stock, par value $.01 per share
("Common Stock"), of the Company (the "Shares").  The Shares include an
overallotment option of up to 3,000,000 shares of Common Stock.  This opinion is
being delivered in connection with the Company's Registration Statement on Form
S-11 (No.333-41449) (the "Registration Statement") relating to the registration
of the offering and sale of the Shares under the Securities Act of 1933, as
amended.  Pursuant to that certain United States purchase agreement between the
Company and the underwriters named below (the "U.S. Purchase Agreement"), up to
18,400,000 of the Shares (including an overallotment option of up to 2,400,000
shares of Common Stock) will be offered by the several United States
underwriters (the "U.S. Underwriters") represented by Goldman, Sachs & Co.,
Merrill Lynch, Pierce, Fenner & Smith Incorporated, Bear, Stearns & Co. Inc.,
Morgan Stanley & Co. Incorporated, PaineWebber Incorporated, Prudential
Securities Incorporated, Smith Barney Inc., Chase Securities Inc., and
Donaldson, Lufkin & Jenrette Securities Corporation.  Pursuant to that certain
international purchase agreement between the Company and the underwriters named
below (together with the U.S. Purchase Agreement, the "Purchase Agreements"), up
to 4,600,000 of the Shares (including an overallotment option of up to 600,000
shares of Common Stock) will be offered by the several international
underwriters (together with the U.S. Underwriters, the "Underwriters")
represented by Goldman Sachs International, Merrill Lynch International, Bear,
Stearns International Limited, Morgan Stanley & Co. International Limited,
PaineWebber International (UK) Ltd., Prudential-Bache Securities (U.K.) Inc.,
Smith Barney Inc., Chase Manhattan International Limited, and Donaldson, Lufkin
& Jenrette International.
<PAGE>
 
                          GOODWIN, PROCTER & HOAR LLP

Boston Properties, Inc.
January 23, 1998
Page 2


     As the basis for the opinion hereinafter expressed, we have examined such
statutes, regulations, corporate records and documents, certificates of public
officials and other instruments as we have deemed necessary or advisable for the
purposes of this opinion.  In such examination, we have assumed the authenticity
of all documents submitted to us as originals and the conformity with the
original documents of all documents submitted to us as copies.

     Based on the foregoing and on such legal considerations as we deem
relevant, we are of the opinion that the Shares to be sold by the Company to the
Underwriters as described in the Registration Statement have been duly
authorized and, upon delivery of such Shares and payment therefor in accordance
with the Purchase Agreements, will be validly issued, fully paid and non-
assessable.

     We hereby consent to the use of this opinion as an exhibit to the
Registration Statement and to the reference to our firm under the heading "Legal
Matters" in the Registration Statement.


                                    Very truly yours,


                                    /s/ Goodwin, Procter & Hoar LLP

                                    Goodwin, Procter & Hoar  LLP
