EXHIBIT 5.01

                                November 13, 2001

Keynote Systems, Inc.
777 Mariners Island Boulevard
San Mateo, CA  94404

Ladies and Gentlemen:

         At your request, we have examined the registration statement on Form
S-8 (the "Registration Statement") filed by you with the Securities and Exchange
Commission (the "Commission") on or about November 13, 2001, in connection with
the registration under the Securities Act of 1933, as amended, of 2,571,992
shares of your common stock (the "Stock"), subject to issuance upon exercise of
stock options granted or to be granted under your 1999 Equity Incentive Plan
(the "Incentive Plan"), and of 514,399 shares of Stock subject to purchase
rights granted or to be granted under your 1999 Employee Stock Purchase Plan
(the "Purchase Plan" and together with the Incentive Plan, the "Plans").

         In rendering this opinion, we have examined the following:

         (1)      your Amended and Restated  Certificate of Incorporation,
                  certified by the Delaware Secretary of State on January 12,
                  2000;

         (2)      your bylaws, certified by your Secretary on February 29, 2000;

         (3)      the Registration Statement, together with the exhibits filed
                  as a part thereof or incorporated therein by reference,
                  including the Incentive Plan and related forms of stock option
                  agreement and stock option exercise agreement, and the
                  Purchase Plan and related forms of enrollment form,
                  subscription agreement, notice of withdrawal and notice of
                  suspension;

         (4)      the registration  statement on Form S-8 (File No.  333-87791)
                  filed by you with the Commission on September 24, 1999,
                  together with the exhibits filed as a part thereof or
                  incorporated therein by reference;

         (5)      the prospectuses prepared in connection with the Registration
                  Statement;

         (6)      the  minutes of  meetings  and  actions  by written  consent
                  of your  stockholders  and Board of  Directors  that are
                  contained  in your minute  books and the minute  books of your
                  predecessor,  Keynote  Systems,  Inc.,  a  California
                  corporation, that are in our possession;

         (7)      a certificate from your transfer agent of even date herewith
                  verifying the number of issued and outstanding shares of your
                  capital stock as of the date hereof and a list of option
                  holders respecting your capital stock and of any rights to
                  purchase capital stock that was prepared by you and dated
                  November 12, 2001 verifying the number of such issued and
                  outstanding securities; and

         (8)      a Management  Certificate  addressed to us and dated of even
                  date herewith executed by you containing certain factual
                  and other  representations.

         In our examination of documents for purposes of this opinion, we have
assumed, and express no opinion as to, the genuineness of all signatures on
original documents, the authenticity and completeness of all documents submitted
to us as originals, the conformity to originals and completeness of all
documents submitted to us as copies, the legal capacity of all persons or
entities executing the same, the lack of any undisclosed termination,
modification, waiver or amendment to any document reviewed by us and the due
authorization, execution and delivery of all documents where due authorization,
execution and delivery are prerequisites to the effectiveness thereof.





         As to matters of fact relevant to this opinion, we have relied solely
upon our examination of the documents referred to above and the representations
and warranties made by you to us, including, but not limited to, those set forth
in the Management Certificate and have assumed the current accuracy and
completeness of the information obtained from the documents referred to above.
We have made no independent investigation or other attempt to verify the
accuracy of any of such information or to determine the existence or
non-existence of any other factual matters; however, we are not aware of any
facts that would cause us to believe that the opinion expressed herein is not
accurate.

         We are admitted to practice law in the State of California, and we
render this opinion only with respect to, and express no opinion herein
concerning the application or effect of the laws of any jurisdiction other than,
the existing laws of the United States of America, of the State of California
and, with respect to the validity of corporate action and the requirements for
the issuance of stock, of the State of Delaware.

         Based upon the foregoing, it is our opinion that the up to 3,086,391
shares of Stock that may be issued and sold by you upon exercise of (a) stock
options granted or to be granted under the Incentive Plan and (b) purchase
rights granted or to be granted under the Purchase Plan, when issued, sold and
delivered in accordance with the applicable plan and purchase agreements to be
entered into thereunder, and in the manner and for the consideration stated in
the Registration Statement and/or the relevant prospectus, will be validly
issued, fully paid and nonassessable.

         We consent to the use of this opinion as an exhibit to the Registration
Statement and further consent to all references to us, if any, in the
Registration Statement, the prospectuses constituting a part thereof and any
amendments thereto. This opinion speaks only as of its date and we assume no
obligation to update this opinion should circumstances change after the date
hereof. This opinion is intended solely for use in connection with the issuance
and sale of shares subject to the Registration Statement and is not to be relied
upon for any other purpose.

                                          Very truly yours,

                                          /s/  Fenwick & West LLP

                                          FENWICK & WEST LLP