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                                                                     EXHIBIT 5.1

                [PATTERSON, BELKNAP, WEBB & TYLER LLP LETTERHEAD]



                               September 29, 1999




Dime Bancorp, Inc.
589 Fifth Avenue
New York, New York 10017

Dear Sirs:

              We have acted as counsel to Dime Bancorp, Inc., a Delaware
corporation (the "Company"), in connection with the proposed registration by the
Company under the Securities Act of 1933, as amended (the "Act"), of 300,000
shares (the "Shares") of the Company's Common Stock, par value $.01 per share
(the "Common Stock"), pursuant to the Company's registration statement on Form
S-8 (the "Registration Statement") filed with the Securities and Exchange
Commission (the "Commission") on the date hereof.

              In rendering this opinion we have examined the Company's
Certificate of Incorporation and Bylaws, each as amended to date, and the
minutes of the corporate proceedings taken by the Company in connection with the
authorization of the Shares. We have also examined the originals, or copies
certified or otherwise identified to us, of the corporate records of the
Company, certificates of public officials and representatives of the Company,
and such other documents and records, and have made such investigations of law,
as we have deemed necessary for purposes of this opinion. We have assumed the
genuineness of all signatures, the conformity to the original of all copies and
the factual accuracy of all certificates submitted to us.

              On the basis of the foregoing, we are of the opinion that the
Shares have been duly authorized by all necessary corporate action on the part
of the Company and when sold and delivered as contemplated by the Registration
Statement will constitute duly authorized, validly issued, fully paid and
nonassessable shares of Common Stock.



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September 29, 1999
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              We express no opinion as to laws other than the corporate laws of
the State of Delaware and the laws of the United States of America.

              We hereby consent to the filing of this opinion with the
Commission as an exhibit to the Registration Statement. In furnishing this
opinion and giving this consent, we do not admit that we are in the category of
persons whose consent is required under Section 7 of the Act, or the rules and
regulations of the Commission thereunder.


                                   PATTERSON, BELKNAP, WEBB & TYLER LLP



                                   By:  /s/ Jeffrey E. LaGueux
                                      --------------------------
                                         A Member of the Firm