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): January 26, 2015

ENERGY TRANSFER PARTNERS, L.P.

(Exact Name of Registrant as Specified in Charter)

3738 Oak Lawn Avenue

Dallas, Texas 75219

(Address of principal executive office) (Zip Code)

(214) 981-0700

(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:

On January 26, 2015, Energy Transfer Partners, L.P. (“ETP”) and Regency Energy Partners LP (“Regency”) issued a press release announcing their entry into a definitive merger agreement pursuant to which Regency will merge with and into ETP, with ETP as the surviving entity. Under the terms of the definitive merger agreement, holders of Regency common units will receive 0.4066 common units of ETP and $0.32 in cash for each Regency common unit. The full text of the press release is included as Exhibit 99.1 to this Current Report on Form 8-K and is incorporated into this Item 7.01 by reference.

In accordance with General Instruction B.2 of Form 8-K, the information set forth in this Item 7.01 and the attached Exhibit 99.1 shall not be deemed “filed” for purposes of Section 18 of the Securities Exchange Act of 1934, as amended.

To the extent required, the information included in Item 7.01 of this Form 8-K is incorporated into this Item 8.01.

Cautionary Statement Regarding Forward-Looking Statements

This report includes “forward-looking” statements. Forward-looking statements are identified as any statement that does not relate strictly to historical or current facts. Statements using words such as “anticipate,” “believe,” “intend,” “project,” “plan,” “expect,” “continue,” “estimate,” “goal,” “forecast,” “may” or similar expressions help identify forward-looking statements. ETP and Regency cannot give any assurance that expectations and projections about future events will prove to be correct. Forward-looking statements are subject to a variety of risks, uncertainties and assumptions. These risks and uncertainties include the risks that the proposed transaction may not be consummated or the benefits contemplated therefrom may not be realized. Additional risks include: the ability to obtain requisite regulatory and unitholder approval and the satisfaction of the other conditions to the consummation of the proposed transaction, the ability of ETP to successfully integrate Regency’s operations and employees and realize anticipated synergies and cost savings, the potential impact of the announcement or consummation of the proposed transaction on relationships, including with employees, suppliers, customers, competitors and credit rating agencies, the ability to achieve revenue, DCF and EBITDA growth, and volatility in the price of oil, natural gas, and natural gas liquids. Actual results and outcomes may differ materially from those expressed in such forward-looking statements. These and other risks and uncertainties are discussed in more detail in filings made by ETP and Regency with the Securities and Exchange Commission (the “SEC”), which are available to the public. ETP and Regency undertake no obligation to update publicly or to revise any forward-looking statements, whether as a result of new information, future events or otherwise.

Additional Information and Where to Find It

SECURITY HOLDERS ARE URGED TO READ THE PROXY STATEMENT/PROSPECTUS AND THE REGISTRATION STATEMENT REGARDING THE TRANSACTION CAREFULLY WHEN IT BECOMES AVAILABLE. These documents (when they become available), and any other documents filed by ETP or Regency with the SEC, may be obtained free of charge at the SEC’s website, at www.sec.gov. In addition, investors and security holders will be able to obtain free copies of the registration statement and the proxy statement/prospectus by phone, e-mail or written request by contacting the investor relations department of ETP or Regency at the following:

2

Participants in the Solicitation

ETP, Regency and their respective directors and executive officers, may be deemed to be participants in the solicitation of proxies in connection with the proposed merger. Information regarding the directors and executive officers of ETP is contained in ETP’s Form 10-K for the year ended December 31, 2013, which was filed with the SEC on February 27, 2014. Information regarding the directors and executive officers of Regency is contained in Regency’s Form 10-K for the year ended December 31, 2013, which was filed with the SEC on February 27, 2014. Additional information regarding the interests of participants in the solicitation of proxies in connection with the proposed merger will be included in the proxy statement/prospectus.

Item 9.01 Financial Statements and Exhibits.

3

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.

4

EXHIBIT INDEX

5