<PAGE>
                                                                    EXHIBIT 10.6

                            FIRST AMENDMENT TO LEASE

      THIS FIRST AMENDMENT TO LEASE (the "First Amendment") is made and entered
into this 30 day of December, 1999 by and between PINE STREET INVESTORS I,
L.L.C., a Delaware limited liability company ("Landlord") and ART TECHNOLOGY
GROUP, INC., a Delaware corporation ("Tenant").

                                    RECITALS

A. Landlord and Tenant have previously entered into that certain written Office
Lease, dated October 6, 1999 ( the "Lease"), pursuant to which Landlord leased
to Tenant, and Tenant hired from Landlord, those certain premises comprising
12,931 square feet of rentable area on the 21st floor (the "Premises") in the
office building located at 100 Pine Street Center, San Francisco, California
(the "Building"). Concurrently with the execution of the Lease, Landlord and
Tenant executed a Work Agreement setting forth the terms and conditions under
which initial tenant improvements would be made to the Premises (the "Work
Agreement").

B. Tenant's proposed Plans under the Work Agreement include non-building
standard ceiling lighting fixtures. As a condition to Landlord's approval of
said Plans, Tenant has agreed to pay for the cost of restoring the Premises to a
building standard condition in accordance with the terms and conditions set
forth herein.

C. The parties desire to amend the Lease upon the terms and conditions set forth
herein.

      NOW, THEREFORE, in consideration of the foregoing, Landlord and Tenant
hereby agree as follows:

      1. All capitalized terms not defined herein shall have the meanings
ascribed to them in the Lease or the Work Agreement, as applicable.

      2. The parties acknowledge and agree that Landlord has agreed to the
installation of non-Building standard lighting fixtures in connection with the
Work performed under the Work Agreement, upon the condition that Tenant pay
Landlord for the cost of the restoration of the Premises ceiling to a
Building-standard condition. Notwithstanding anything to the contrary in the
Lease or the Work Agreement (including, without limitation, Article 22 of the
Lease), Tenant hereby agrees to pay for the cost of: (i) removing the ceiling
light fixtures installed as part of the


                                       1
<PAGE>
initial improvements to the Premises under the Work Agreement and (ii) restoring
the Premises ceiling lights and ceiling grid (including tiles) to a building
standard condition (the "Ceiling Restoration"). The parties hereby agree that
the cost of the Ceiling Restoration is equal to $44,218.00 (the "Ceiling
Restoration Cost"). The Ceiling Restoration Cost shall be paid by Tenant by
means of a Letter of Credit (defined below) upon which Landlord will have the
right to draw upon the expiration or earlier termination of the Lease.

      3. Tenant shall deliver and cause to be in effect upon the execution of
this First Amendment, during the Lease Term, and for a period sixty (60) days
after the Expiration Date, an unconditional, irrevocable letter of credit in the
amount of $44,218.00 in favor of Landlord ("Letter of Credit"). The Letter of
Credit shall serve as a means of payment for the Ceiling Restoration Costs. In
accordance with the terms hereof, Tenant shall deliver the Letter of Credit to
Landlord upon the execution of this First Amendment. The Letter of Credit shall
be in a form reasonably acceptable to Landlord and issued by a bank selected by
Tenant and reasonably acceptable to Landlord. Without limiting the generality of
the foregoing, any bank issuing the Letter of Credit shall be a bank that
accepts deposits, maintains accounts, has a local San Francisco office that will
negotiate a letter of credit, and the deposits of which are insured by the
Federal Deposit Insurance Corporation. Without limiting the generality of the
foregoing, the Letter of Credit shall provide: (i) that Landlord shall have the
right to draw upon such letter of credit immediately following the expiration or
earlier termination of the Lease, and (ii) that the issuing bank shall deliver
to Landlord at least sixty (60) days prior written notice of the termination or
expiration of any such Letter of Credit. Tenant shall pay all expenses, points,
or fees incurred by Tenant in obtaining and maintaining the Letter of Credit.
Tenant agrees, upon termination of the Lease, that Landlord shall have the right
to draw upon all of the Letter of Credit to compensate Landlord for the Ceiling
Restoration Cost. The failure by Tenant to provide Landlord with the Letter of
Credit or the failure by Tenant to provide a substitute Letter of Credit meeting
all of the criteria set forth herein on or before thirty (30) days prior to the
expiration or termination of any prior Letter of Credit shall constitute a
material breach of the Lease and be deemed a Default thereunder.

      4. Except as amended hereby, the Lease and the Work Agreement remain
unmodified and in full force and effect. Tenant acknowledges that Landlord is
not in default in the performance of any of its obligations under the Lease and
that Tenant has no claims or setoffs of any kind.


                                       2
<PAGE>
      IN WITNESS WHEREOF, Landlord and Tenant have duly executed this First
Amendment as of the date and year first above written.


LANDLORD:

PINE STREET INVESTORS I L.L.C., a
Delaware Limited Liability Company

By:     Danielson Whitehead, Inc.
        a California corporation,
        as agent for Pine Street
        Investors I, L.L.C.

        By:     /s/ Carl F. Danielson
                -----------------------
        Name:   Carl F. Danielson
                -----------------------
        Title:  Chief Executive Officer
                -----------------------


TENANT:

ART TECHNOLOGY GROUP, INC.,
a Delaware corporation

By: /s/ Paul Shorthose
   --------------------------------
Name:
     ------------------------------
Title:
      -----------------------------


                                       3