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                                                                   EXHIBIT 10.26

                        FIFTH LOAN MODIFICATION AGREEMENT

      This Fifth Loan Modification Agreement (this "Loan Modification
Agreement') is entered into as of June ___, 2004, by and between SILICON VALLEY
BANK, a California-chartered bank, with its principal place of business at 3003
Tasman Drive, Santa Clara, California 95054 and with a loan production office
located at One Newton Executive Park, Suite 200, 2221 Washington Street, Newton,
Massachusetts 02462, doing business under the name "Silicon Valley East"
("Bank") and ART TECHNOLOGY GROUP, INC., a Delaware corporation with its
principal place of business at 25 First Street, Cambridge, Massachusetts 02141
("Borrower").

1.    DESCRIPTION OF EXISTING INDEBTEDNESS AND OBLIGATIONS. Among other
      indebtedness and obligations which may be owing by Borrower to Bank,
      Borrower is indebted to Bank pursuant to a loan arrangement dated as of
      June 13, 2002, evidenced by, among other documents, a certain Amended and
      Restated Loan and Security Agreement dated as of June 13, 2002, between
      Borrower and Bank, as amended by a certain First Loan Modification
      Agreement dated as of September 27, 2002, as further amended by a certain
      Amendment dated as of October __, 2002, as further amended by a certain
      Second Loan Modification Agreement dated as of December 24, 2002, and as
      amended by a certain Third Loan Modification Agreement dated as of October
      20, 2003, and as amended by a certain Fourth Loan Modification Agreement
      dated November 26, 2003, and as further amended by a certain Letter
      Agreement dated June 16, 2004 (as amended, the "Loan Agreement").
      Capitalized terms used but not otherwise defined herein shall have the
      same meaning as in the Loan Agreement.

2.    DESCRIPTION OF COLLATERAL. Repayment of the Obligations is secured by the
      Collateral as described in the Loan Agreement (together with any other
      collateral security granted to Bank, the "Security Documents").

Hereinafter, the Security Documents, together with all other documents
evidencing or securing the Obligations shall be referred to as the "Existing
Loan Documents".

3.    DESCRIPTION OF CHANGE IN TERMS.

      A. Modifications to Loan Agreement.

            1.    The Loan Agreement shall be amended by deleting the following
                  text, appearing in Section 6.7(b) thereof:

                        "(b) PROFITABILITY. Borrower shall have quarterly net
                        losses of not more than (i) $3,500,000.00 for the
                        quarter ending December 31, 2003; (ii) $3,000,000.00 for
                        the quarter ending March 31, 2004; (iii) $2,000,000.00
                        for the quarter ending June 30, 2004; (iv) $2,000,000.00
                        for the quarter ending September 30, 2004; (v)
                        $1,000,000.00 for each quarter thereafter."

                  and inserting in lieu thereof the following:

                        "(b) PROFITABILITY. Borrower shall have quarterly net
                        losses of not more than (i) $3,500,000.00 for the
                        quarter ending December 31, 2003; (ii) $3,000,000.00 for
                        the quarter ending March 31, 2004; (iii) $5,000,000.00
                        for the quarter ending June 30, 2004; (iv) $2,000,000.00
                        for the quarter ending September 30, 2004; (v)
                        $1,000,000.00 for each quarter thereafter."

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            4.    FEES. Borrower shall pay to Bank a modification fee equal to
                  Five Thousand Dollars ($5,000.00), which fee shall be due on
                  the date hereof and shall be deemed fully earned as of the
                  date hereof. The Borrower shall also reimburse Bank for all
                  reasonable legal fees and expenses incurred in connection with
                  this amendment to the Existing Loan Documents.


            5.    RATIFICATION OF INTELLECTUAL PROPERTY SECURITY AGREEMENT.
                  Borrower hereby ratifies, confirms and reaffirms, all and
                  singular, the terms and conditions of a certain Intellectual
                  Property Security Agreement dated as of June 13, 2002 between
                  Borrower and Bank, and acknowledges, confirms and agrees that
                  said Intellectual Property Security Agreement contains an
                  accurate and complete listing of all Intellectual Property
                  Collateral as defined in said Intellectual Property Security
                  Agreement and shall remain in full force and effect.

            6.    RATIFICATION OF PERFECTION CERTIFICATE. Borrower hereby
                  ratifies, confirms and reaffirms, all and singular, the terms
                  and disclosures contained in a certain Perfection Certificate
                  dated as of June 13, 2002 between Borrower and Bank, and
                  acknowledges, confirms and agrees the disclosures and
                  information Borrower provided to Bank in said Perfection
                  Certificate has not changed, as of the date hereof.

            7.    CONSISTENT CHANGES. The Existing Loan Documents are hereby
                  amended wherever necessary to reflect the changes described
                  above.

            8.    RATIFICATION OF LOAN DOCUMENTS. Borrower hereby ratifies,
                  confirms, and reaffirms all terms and conditions of all
                  security or other collateral granted to the Bank, and confirms
                  that the indebtedness secured thereby includes, without
                  limitation, the Obligations.

            9.    NO DEFENSES OF BORROWER. Borrower hereby acknowledges and
                  agrees that Borrower has no offsets, defenses, claims, or
                  counterclaims against Bank with respect to the Obligations, or
                  otherwise, and that if Borrower now has, or ever did have, any
                  offsets, defenses, claims, or counterclaims against Bank,
                  whether known or unknown, at law or in equity, all of them are
                  hereby expressly WAIVED and Borrower hereby RELEASES Bank from
                  any liability thereunder.

            10.   CONTINUING VALIDITY. Borrower understands and agrees that in
                  modifying the existing Obligations, Bank is relying upon
                  Borrower's representations, warranties, and agreements, as set
                  forth in the Existing Loan Documents. Except as expressly
                  modified pursuant to this Loan Modification Agreement, the
                  terms of the Existing Loan Documents remain unchanged and in
                  full force and effect. Bank's agreement to modifications to
                  the existing Obligations pursuant to this Loan Modification
                  Agreement in no way shall obligate Bank to make any future
                  modifications to the Obligations. Nothing in this Loan
                  Modification Agreement shall constitute a satisfaction of the
                  Obligations. It is the intention of Bank and Borrower to
                  retain as liable parties all makers of Existing Loan
                  Documents, unless the party is expressly released by Bank in
                  writing. No maker will be released by virtue of this Loan
                  Modification Agreement.

            11.   COUNTERSIGNATURE. This Loan Modification Agreement shall
                  become effective only when it shall have been executed by
                  Borrower and Bank (provided, however, in no event shall this
                  Loan Modification Agreement become effective until signed by
                  an officer of Bank in California).

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      This Loan Modification Agreement is executed as a sealed instrument under
the laws of the Commonwealth of Massachusetts as of the date first written
above.

BORROWER:                                     BANK:

ART TECHNOLOGY GROUP, INC.                    SILICON VALLEY BANK, doing
                                              business as SILICON VALLEY EAST

By: /s/ Edward Terino                        By: /s/ Irina Case
    -------------------------                    -------------------------------
    Name: Edward Terino                          Name: Irina Case
    Title: CFO                                   Title: VP


                                              SILICON VALLEY BANK

                                              By: /s/ Maggie Garcia
                                                  ------------------------------
                                                  Name: Maggie Garcia
                                                  Title: AVP
                                                 (signed in Santa Clara County,
                                                  California)