<PAGE>


                       SECURITIES AND EXCHANGE COMMISSION
                             Washington, D.C. 20549

                                    FORM 10-K

  /X/ ANNUAL REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES EXCHANGE
             ACT OF 1934 FOR THE FISCAL YEAR ENDED DECEMBER 31, 2000
                                       OR
     / / TRANSITION REPORT PURSUANT TO SECTION 13 OR 15(d) OF THE SECURITIES
                              EXCHANGE ACT OF 1934
             FOR THE TRANSITION PERIOD FROM __________ TO __________

                        Commission file number 000-26679

                           ART TECHNOLOGY GROUP, INC.
             (Exact name of registrant as specified in its charter)

           Delaware                                      04-3141918
(State or other jurisdiction of                       (I.R.S. Employer
incorporation or organization)                     Identification Number)

                 25 First Street, Cambridge, Massachusetts 02141
                                   Jeet Singh
                            Chief Executive Officer
                           Art Technology Group, Inc.
                                25 First Street
                         Cambridge, Massachusetts 02141
                                 (617) 386-1000

           Securities registered pursuant to Section 12(b) of the Act:
                                      None

           Securities registered pursuant to Section 12(g) of the Act:
                          Common Stock, $0.01 par value
                                (Title of Class)

Indicate by check mark whether the registrant (1) has filed all reports required
to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934
during the preceding 12 months (or for such shorter period that the registrant
was required to file such reports), and (2) has been subject to such filing
requirements for the past 90 days. Yes: |X| No: |_|

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405
of Regulation S-K is not contained herein, and will not be contained, to the
best of registrant's knowledge, in definitive proxy or information statements
incorporated by reference in Part III of this Form 10-K or any attachment to
this Form 10-K. |_|

The aggregate market value of the voting stock held by non-affiliates of the
registrant was approximately $1,366,309,680 based on the closing price of the
Common Stock on the NASDAQ National Market on March 26, 2001.

The number of shares of the registrant's Common Stock outstanding as of
March 26, 2001, was 68,315,484.

                       Documents Incorporated by Reference

       DOCUMENT DESCRIPTION                                           10-K PART
       --------------------                                           ---------
Portions of the Registrant's Proxy Statement for the Annual
   Meeting of Stockholders to be held May 14, 2001.................      III


<PAGE>


                               EXPLANATORY NOTE

Due to an oversight, the number of shares outstanding on March 26, 2001 and
the market value of the voting stock on that date were omitted from the
original filing.

<PAGE>


                                   SIGNATURES

Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange
Act of 1934, the registrant has duly caused this report to be signed on its
behalf by the undersigned, thereunto duly authorized, as of April 6, 2001.

ART TECHNOLOGY GROUP, INC.
(Registrant)


By:    /s/ Jeet Singh
       --------------
       Jeet Singh
       Chief Executive Officer (Principal Executive Officer)



By:    /s/ Ann C. Brady
       ----------------
       Ann C. Brady
       Vice President, Finance and Chief Financial Officer
       (Principal Financial and Accounting Officer)



Pursuant to the requirements of the Securities Exchange Act of 1934, this report
has been signed below by the following persons on behalf of the registrant and
in the capacities as of April 6, 2001.

            NAME                   TITLE
            ----                   -----


   /s/ Phyllis S. Swersky         Director
   ----------------------
   Phyllis S. Swersky


   /s/ Scott A. Jones             Director
   ------------------
   Scott A. Jones


   /s/ Charles R. Lax             Director
   ------------------
   Charles R. Lax


   /s/ Thomas N. Matlack          Director
   ---------------------
   Thomas N. Matlack


   /s/ Robert F. Walters          Director
   ---------------------
   Robert F. Walters


   /s/ Joseph T. Chung            Chairman of the Board
   ---------------------
   Joseph T. Chung


   /s/ Jeet Singh                 Director and Chief Executive Officer
   ---------------------
   Jeet Singh