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): October 15, 2015

TARGA RESOURCES PARTNERS LP

(Exact name of registrant as specified in its charter)

1000 Louisiana, Suite 4300

Houston, TX 77002

(Address of principal executive office and Zip Code)

(713) 584-1000

(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:

Item 3.03. Material Modification to Rights of Security Holders.

The information set forth under Item 5.03 is incorporated by reference into this Item 3.03.

Item 5.03. Amendments to Articles of Incorporation or Bylaws; Change in Fiscal Year.

On October 15, 2015, Targa Resources Partners LP (the “Partnership”) executed the Second Amended and Restated Agreement of Limited Partnership of Targa Resources Partners LP (the “Partnership Agreement”) for the purpose of defining the preferences, rights, powers and duties of holders of 9.00% Series A Fixed-to-Floating Rate Cumulative Redeemable Perpetual Preferred Units (the “Preferred Units”) in connection with the offering (the “Offering”) of Preferred Units that closed on October 15, 2015. The Partnership Agreement also incorporates, among other things, (i) the duly authorized amendments to the Partnership Agreement previously approved by the board of directors of the general partner of the Partnership (the “Board”), (ii) revisions to certain tax provisions to further conform these provisions to applicable Treasury Regulations and the manner in which the major accounting firms apply these provisions, and (iii) changes to the Partnership Agreement to reflect the passage of time and to remove provisions that are no longer applicable.

The Preferred Units rank senior to the Partnership’s common units, with respect to the payment of distributions and distribution of assets upon liquidation, dissolution and winding up. The Preferred Units have no stated maturity and are not subject to mandatory redemption or any sinking fund and will remain outstanding indefinitely unless repurchased or redeemed by the Partnership or converted into its common units in connection with a change of control.

Holders of Preferred Units issued in the Offering will be entitled to receive, when, as and if declared by the Board, out of legally available funds for such purposes, cumulative cash monthly distributions. Distributions on Preferred Units will be cumulative from the date of issue and payable monthly on the 15^th day of each month, commencing November 15, 2015. Distributions on the Preferred Units will be payable from and including the date of original issue to, but not including, November 1, 2020, at a rate equal to 9.00% per annum of the stated liquidation preference. On and after November 1, 2020, distributions on the Preferred Units will accumulate at an annual floating rate equal to the one-month LIBOR plus a spread of 7.71%.

At any time on or after November 1, 2020, the Partnership may redeem the Preferred Units, in whole or in part, from any source of funds legally available for such purpose, by paying $25.00 per unit plus an amount equal to all accumulated and unpaid distributions thereon to the date of redemption, whether or not declared. In addition, the Partnership (or a third-party with its prior written consent) may redeem the Preferred Units following certain changes of control, as described in the Partnership Agreement. If the Partnership (or a third-party with its prior written consent) does not exercise this option, then the holders of the Preferred Units have the option to convert the Preferred Units into a number of common units per Preferred Unit as set forth in the Partnership Agreement. If the Partnership (or a third-party with its prior written consent) exercises its redemption rights relating to any Preferred Units, the holders of those Preferred Units will not have the conversion right described above with respect to the Preferred Units called for redemption.

Holders of Preferred Units will have no voting rights except for limited voting rights with respect to potential amendments to the Partnership Agreement that have a material adverse effect on the existing terms of the Preferred Units and in certain other limited circumstances or as required by law. The description of the Partnership Agreement contained in this Item 5.03 is qualified in its entirety by reference to the full text of the Partnership Agreement, which is filed as Exhibit 3.1 hereto and is incorporated by reference herein.

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Item 9.01 Financial Statements and Exhibits.

(d) Exhibits

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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 thereunto duly authorized.

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EXHIBIT INDEX

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