UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

WASHINGTON, D.C. 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): July 1, 2012

PROSPERITY BANCSHARES, INC.

(Exact name of registrant as specified in its charter)

Registrant’s telephone number, including area code: (281) 269-7199

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 (see General Instruction A.2. below):

On July 1, 2012, the previously announced merger (the “Merger”) of American State Financial Corporation (“American State”), a Texas corporation and parent company of American State Bank, Lubbock, Texas, with and into Prosperity Bancshares, Inc. (“Prosperity”), a Texas corporation and parent company of Prosperity Bank, El Campo, Texas, pursuant to the terms of an Agreement and Plan of Reorganization dated February 26, 2012 (the “Agreement”) was completed. Immediately following the Merger, American State Bank was merged with and into Prosperity Bank.

In connection with the Merger, Prosperity issued approximately 8,525,000 shares of Prosperity common stock and paid approximately $178.5 million in cash to shareholders of American State in exchange for all outstanding shares of American State capital stock.

The press release issued by Prosperity announcing the Merger is attached as Exhibit 99.1 to this Current Report on Form 8-K

Pursuant to the terms of the Agreement, the Board of Directors of Prosperity appointed W. R. Collier, who was a member of the board of directors of American State prior to the consummation of the Merger, to the Board of Directors of Prosperity, with such appointment to be effective as of the first Board meeting to be held following completion of the Merger, which is scheduled for August 21, 2012. Mr. Collier was appointed to fill a vacancy as a Class II director and will serve until Prosperity’s 2013 annual meeting of shareholders. As of the date of this filing, Mr. Collier has not been appointed to a committee of Prosperity’s Board of Directors, nor has it been determined when, if at all, any such appointment would be made. Mr. Collier will receive fees for his service as a director consistent with those fees received by the existing directors for service as a director of the Company.

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(d) Exhibits

The following are filed as exhibits to this Current Report on Form 8-K:

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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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