UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, DC 20549

FORM 8-K

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 8, 2013

OPTIMER PHARMACEUTICALS, INC.

(Exact name of registrant as specified in its charter)

101 Hudson Street, Suite 3501, Jersey City, NJ 07302

(Address of principal executive offices, including zip code)

(201) 333-8819

(Registrant’s telephone number, including area code)

N/A

(Former name or former address, if changed since last report)

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:

o Written communications pursuant to Rule 425 under the Securities Act (17 CFR 230.425)

o Soliciting material pursuant to Rule 14a-12 under the Exchange Act (17 CFR 240.14a-12)

o Pre-commencement communications pursuant to Rule 14d-2(b) under the Exchange Act (17 CFR 240.14d-2(b))

o Pre-commencement communications pursuant to Rule 13e-4(c) under the Exchange Act (17 CFR 240.13e-4(c))

Item 2.02.     Results of Operations and Financial Condition.

On May 9, 2013, Optimer Pharmaceuticals, Inc. (the “Company”) issued a press release announcing its unaudited financial results for the quarter ended March 31, 2013.  A copy of this press release is attached hereto as Exhibit 99.1.

This information included in this Item 2.02 and the exhibit hereto are being furnished and shall not be deemed to be “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended (the “Exchange Act”), or otherwise subject to the liability of that section, nor shall they be deemed incorporated by reference in any filing under the Securities Act of 1933, as amended, or the Exchange Act, except as expressly set forth by specific reference in such a filing.

Item 5.07.     Submission of Matters to a Vote of Security Holders.

Set forth below are the results of the matters submitted for a vote of stockholders at the Company’s 2013 Annual Meeting of Stockholders held on May 8, 2013.

Proposal 1 — Election of Directors

The following directors were elected to serve for three-year terms until the 2016 Annual Meeting of Stockholders and until their respective successors are elected and qualified.

Proposal 2 — Ratification of the appointment of Ernst and Young, LLP as our independent registered public accounting firm for the fiscal year ending December 31, 2013.

Proposal 3 — Approval, on and advisory basis, of the compensation of our named executive officers.

Item 9.01.     Financial Statements and Exhibits.

(d)  Exhibits.

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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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EXHIBIT INDEX

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