EXHIBIT 32.1

              CERTIFICATION OF PRINCIPAL EXECUTIVE OFFICER PURSUANT
                TO 18 U.S.C. SECTION 1350, AS ADOPTED PURSUANT TO
                  SECTION 906 OF THE SARBANES-OXLEY ACT OF 2002

      In connection with the Annual Report on Form 10-K of Art Technology Group,
Inc. (the "Company") for the period ended December 31, 2005, as filed with the
Securities and Exchange Commission on the date hereof (the "Report"), the
undersigned, Robert D. Burke, the President and Chief Executive Officer of the
Company, hereby certifies, to his best knowledge and belief, pursuant to 18
U.S.C. Section 1350, as adopted pursuant to Section 906 of the Sarbanes-Oxley
Act of 2002, that:

            (1) the Report fully complies with the requirements of Section 13(a)
            or 15(d) of the Securities Exchange Act of 1934; and

            (2) the information contained in the Report fairly presents, in all
            material respects, the financial condition and results of operations
            of the Company.

                                                 /s/ ROBERT D. BURKE
                                        ----------------------------------------
                                                      Robert D. Burke
                                          President and Chief Executive Officer
                                              (principal executive officer)

Date: March 13, 2006