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

January 8, 2013

Date of Report (Date of earliest event reported)

ATMOS ENERGY CORPORATION

(Exact Name of Registrant as Specified in its Charter)

(972) 934-9227

(Registrant’s Telephone Number, Including Area Code)

Not Applicable

(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:

On January 8, 2013, Atmos Energy Corporation (“Atmos Energy”) entered into an underwriting agreement (the “Underwriting Agreement”) with J.P. Morgan Securities LLC, Mitsubishi UFJ Securities (USA), Inc. and U.S. Bancorp Investments, Inc., as representatives of the several underwriters named in Schedule I thereto, with respect to the offering and sale in an underwritten public offering (the “Offering”) by Atmos Energy of $500 million aggregate principal amount of its 4.15% Senior Notes due 2043 (the “Notes”), with a yield to maturity of 4.161% and an effective yield to maturity of 4.636%, after giving effect to the settlement of related Treasury lock agreements. The Offering has been registered under the Securities Act pursuant to a registration statement on Form S-3 (Registration No. 333-165818) of Atmos Energy (the “Registration Statement”), and the prospectus supplement dated January 8, 2013, which was filed with the Securities and Exchange Commission pursuant to Rule 424(b) of the Securities Act on January 9, 2013. Legal opinions related to the Registration Statement are also filed herewith as Exhibits 5.1 and 5.2.

Atmos Energy expects to receive net proceeds, after the underwriting discount and estimated offering expenses, of approximately $494 million. The Offering is expected to close on January 11, 2013, subject to customary closing conditions.

The Notes will be issued pursuant to an indenture dated March 26, 2009 (the “Indenture”) between Atmos Energy and U.S. Bank, National Association, as trustee (the “Trustee”), to be modified by an Officers’ Certificate setting forth the terms of the Notes (the “Officers’ Certificate”), to be dated January 11, 2013 and delivered to the Trustee pursuant to Section 301 of the Indenture. The Notes will be represented by a global security, a form of which is filed as an exhibit hereto. The form of Officers’ Certificate and the Underwriting Agreement are each also filed as an exhibit hereto.

SIGNATURE

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.

INDEX TO EXHIBITS