UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 8-K/A

(Amendment No. 1)

CURRENT REPORT

PURSUANT TO SECTION 13 OR 15(d)

OF THE SECURITIES EXCHANGE ACT OF 1934

Date of Report (Date of earliest event reported): May 1, 2013

XENOPORT, INC.

(Exact name of registrant as specified in its charter)

3410 Central Expressway

Santa Clara, California 95051

(Address of Principal Executive Offices) (Zip Code)

Registrant’s telephone number, including area code: (408) 616-7200

Check the appropriate box below if the Form 8-K filing is intended to simultaneously satisfy the filing obligation of the registrant under any of the following provisions:

EXPLANATORY NOTE

On May 6, 2013, XenoPort, Inc. (the “Company” or “XenoPort”) filed a Current Report on Form 8-K (the “Original Report”) reporting that on May 1, 2013, pursuant to a Termination and Transition Agreement entered into between the Company and Glaxo Group Limited (“GSK”) on November 8, 2012 (the “Termination Agreement”), XenoPort completed the reacquisition of the exclusive rights to commercialize, promote, manufacture and distribute Horizant^® (gabapentin enacarbil) Extended-Release Tablets in the United States from GSK and assumed all responsibilities for any further development, manufacturing and commercialization of Horizant in the United States beginning on that date.

This Current Report on Form 8-K/A amends the above-referenced Original Report. In the Original Report, XenoPort indicated that it would file any financial statements and pro forma financial information required under Items 9.01(a) and (b) of Form 8-K not later than 71 days after the date on which the Original Report was required to be filed. By filing this Current Report on Form 8-K/A, XenoPort is amending and restating Item 9.01 of the Original Report and amending the Original Report to provide the below-identified financial statements, pro forma financial information and consents.

Item 9.01 Financial Statements and Exhibits.

XenoPort has been advised by GSK that it is impracticable to prepare complete financial statements related to the Horizant business in order to enable the filing of such financial statements as required by Rule 3-05 of Regulation S-X in connection with XenoPort’s reacquisition of the rights to Horizant. In this regard, XenoPort has been advised by GSK that the Horizant product line was not a separate legal entity of GSK and was never operated as a stand-alone business, division or subsidiary. GSK has also advised the Company that it has never prepared full stand-alone or full carve-out financial statements for the Horizant business, and that GSK has never maintained the distinct and separate accounts necessary to prepare such financial statements. As a result, XenoPort is filing the following financial statements related to the Horizant business for the purpose of complying with the requirements of Rule 3-05 of Regulation S-X:

Pursuant to a letter dated June 4, 2013 from the staff of the Division of Corporation Finance (the “Division”) of the Securities and Exchange Commission (the “Commission”), the Division stated that it will not object to the Company’s conclusion that the filing of the above-identified financial statements represents substantial compliance with the requirements of Rule 3-05 of Regulation S-X.

The unaudited pro forma condensed combined balance sheet as of March 31, 2013 and the unaudited pro forma condensed combined statements of comprehensive loss for the year ended December 31, 2012 and for the three months ended March 31, 2013, and the notes related thereto, in each case reflecting XenoPort’s reacquisition of the Horizant business, are filed as Exhibit 99.3 to this Current Report on Form 8-K/A and are incorporated herein by reference.

SIGNATURES

Pursuant to the requirements of the Securities Exchange Act of 1934, the registrant has duly caused this report to be signed on its behalf by the undersigned hereunto duly authorized.

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