UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

SCHEDULE 14A

(Rule 14a-101)

SCHEDULE 14A INFORMATION

Proxy Statement Pursuant to Section 14(a) of the

Securities Exchange Act of 1934

Filed by the Registrant  x                            Filed by a Party other than the Registrant  ¨

Check the appropriate box:

¨ Preliminary Proxy Statement

¨ Confidential, for Use of the Commission Only (as permitted by Rule 14a-6(e)(2))

¨ Definitive Proxy Statement

x Definitive Additional Materials

¨ Soliciting Material under §240.14a-12

Sonic Corp.

(Name of registrant as specified in its charter)

(Name of person(s) filing proxy statement, if other than the registrant)

Payment of Filing Fee (Check the appropriate box):

x No fee required.

¨ Fee computed on table below per Exchange Act Rules 14a-6(i)(1) and 0-11.

(1) Title of each class of securities to which the transaction applies:

(2) Aggregate number of securities to which the transaction applies:

(3) Per unit price or other underlying value of the transaction computed pursuant to Exchange Act Rule 0-11 (set forth the amount on which the filing fee is calculated and state how it was determined):

(4) Proposed maximum aggregate value of the transaction:

(5) Total fee paid:

¨ Fee paid previously with preliminary materials.

¨ Check box if any part of the fee is offset as provided by Exchange Act Rule 0-11(a)(2) and identify the filing for which the offsetting fee was paid previously. Identify the previous filing by registration statement number, or the Form or Schedule and the date of its filing.

(1) Amount Previously Paid:

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On November 22, 2010, Sonic Corp. (the “Company”) filed and distributed its definitive proxy statement for the annual meeting of stockholders of the Company to be held on January 6, 2011 (the “Proxy Statement”).  In the Proxy Statement, the Company set forth instructions for stockholders wishing to grant their proxy in respect of certain items which will be voted upon at the annual meeting.  This is a reminder to stockholders to return their proxy to the Company and a repetition of the recommendation of the Company’s board of directors in respect of the items to be voted upon.

Carolyn C. Cummins

Vice President and Corporate Secretary

December 15, 2010

Dear Stockholder:

According to our latest records, we have not yet received your proxy for the important Annual Meeting of Sonic Corp. stockholders, to be held on January 6, 2011.  Your Board of Directors recommends that stockholders vote FOR proposals 1 and 2 and AGAINST proposal 3.

Your vote is important, no matter how many or how few shares you may own.  If you have not already done so, please vote TODAY by telephone, via the Internet, or by signing, dating and returning the enclosed proxy card in the envelope provided.

Very truly yours,

Carolyn C. Cummins

Corporate Secretary

REMEMBER:

You can vote your shares by telephone, or via the Internet.

Please follow the easy instructions on the enclosed card.

If you have any questions, or need assistance in voting

your shares, please call our proxy solicitor,

INNISFREE M&A INCORPORATED

TOLL-FREE, at 1-888-750-5834.