UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14D-9

Solicitation/Recommendation Statement under Section 14(d)(4) of the

Securities Exchange Act of 1934

(Amendment No. 11)

Portal Software, Inc.

(Name of Subject Company)

Portal Software, Inc.

(Name of Person(s) Filing Statement)

Common Stock, par value $0.001 per share

(Title of Class of Securities)

736126301

(CUSIP Number of Class of Securities)

Larry Bercovich

Senior Vice President, General Counsel and Secretary

Portal Software, Inc.

10200 South De Anza Boulevard

Cupertino, CA 95014

(Name, address and telephone number of person

authorized to receive notices and communications on

behalf of the person(s) filing statement)

With a copy to:

Michael S. Ringler, Esq.

Wilson Sonsini Goodrich & Rosati

Professional Corporation

One Market, Spear Tower, Suite 3300

San Francisco, CA 94105

(415) 947-2000

This Amendment No. 11 amends and supplements the Solicitation/Recommendation Statement on Schedule 14D-9 initially filed with the Securities and Exchange Commission (the “SEC”) on April 26, 2006, as amended (as previously filed with the SEC, the “Schedule 14D-9”), by Portal Software, Inc., a Delaware corporation (“Portal” or the “Company”), relating to the tender offer made by Potter Acquisition Corporation, a Delaware corporation (“Offeror”), a wholly-owned subsidiary of Oracle Systems Corporation, a Delaware corporation (“Parent”), which is a wholly-owned subsidiary of Oracle Corporation, a Delaware corporation (“Oracle”), as set forth in a Tender Offer Statement filed by Offeror, Parent and Oracle on Schedule TO, dated April 25, 2006, as amended (as previously filed with the SEC, the “Schedule TO”), to pay $4.90 net per share, without interest, less any required withholding taxes, upon the terms and subject to the conditions set forth in the Offer to Purchase, dated April 25, 2006, as amended, and in the related Letter of Transmittal. Any capitalized terms used and not otherwise defined herein shall have the meaning ascribed to such term in the Schedule 14D-9.

All information in the Schedule 14D-9 is incorporated in this Amendment No. 11, except that such information is hereby amended to the extent specifically provided herein.

Item 8 is hereby amended and supplemented as follows:

“The Offer. The subsequent offering period of the Offer expired at 8:00 p.m., New York City time, on Friday, June 30, 2006. According to Computershare Trust Company, N.A., the depositary for the Offer, approximately 38.3 million Shares were validly tendered in the Offer and not withdrawn, representing approximately 88.6% of the outstanding Shares. Offeror has accepted for payment all tendered Shares. Together with the approximately 5.3 million Shares acquired by Oracle, Parent and Offeror through open market purchases at $4.90 per Share and through the exercise of the Top-Up Option, Oracle, Parent and Offeror collectively control 90% of the outstanding Shares.

On July 3, 2006, Oracle also announced that the Merger occurred on July 3, 2006. In connection with the Merger, each outstanding Share not tendered in the Offer (other than Shares held by the Company, Oracle, Parent and Offeror and Shares held by stockholders who properly perfect appraisal rights under Delaware law) was converted into the right to receive $4.90 per share in cash, without interest. Following the consummation of the Merger, the Company will continue as the surviving corporation and be a wholly-owned subsidiary of Parent.”

-2-

SIGNATURE

After due inquiry and to the best of my knowledge and belief, I certify that the information set forth in this statement is true, complete and correct.

-3-