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): May 9, 2013

REGENCY ENERGY PARTNERS LP

(Exact name of Registrant as specified in its charter)

2001 Bryan Street, Suite 3700

Dallas, Texas 75201

(Address of principal executive offices, including zip code)

(214) 750-1771

(Registrant’s telephone number, including area code)

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:

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

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

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

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

Regency Energy Partners LP, a Delaware limited partnership (the “Partnership”), is filing this Current Report on Form 8-K to provide the legal opinion of Latham & Watkins LLP relating to tax matters, a copy of which is filed as Exhibit 8.1 hereto, in connection with the Partnership’s prospectus supplement filed with the Securities and Exchange Commission on May 9, 2013. Such prospectus supplement amends the Partnership’s prospectus supplement dated June 19, 2012 and related base prospectus dated June 8, 2012 to provide updated disclosure regarding the material tax considerations associated with the Partnership’s operations and the purchase, ownership and disposition of the Partnership’s common units, and replaces our prospectus supplement dated March 4, 2013 in its entirety.

(d) Exhibits.

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.

EXHIBIT INDEX