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): March 27, 2013

ACCESS MIDSTREAM PARTNERS, L.P.

(Exact name of Registrant as specified in its Charter)

(405) 935-7800

(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:

Section 1 — Registrant’s Business and Operations

Item 1.01 Entry into a Material Definitive Agreement

Equity Underwriting Agreement

On March 27, 2013, Access Midstream Partners, L.P. (the “Partnership”), Access Midstream Partners GP, L.L.C., (the “General Partner”), and Access MLP Operating, L.L.C. (“MLP Operating,” and together with the Partnership and the General Partner, the “Partnership Parties”) entered into an Underwriting Agreement (the “Underwriting Agreement”) with Barclays Capital Inc., acting as representative of the several underwriters named therein (the “Underwriters”), providing for the offer and sale by the Partnership Parties (the “Offering”), and purchase by the Underwriters, of 9,000,000 common units (the “Firm Units”) representing limited partner interests (the “Common Units”) in the Partnership, at a price to the public of $39.86 per Common Unit. In addition, the Partnership granted to the Underwriters the option for a period of 30 days to purchase up to an additional 1,350,000 Common Units (the “Additional Units”) on the same terms. The material terms of the Offering are described in the prospectus supplement, dated March 27, 2013 (the “Prospectus Supplement”), filed by the Partnership with the United States Securities and Exchange Commission (the “Commission”) on March 28, 2013 pursuant to Rule 424(b)(5) under the Securities Act of 1933, as amended (the “Act”). The Offering was registered with the Commission pursuant to a Registration Statement on Form S-3, as amended (File No. 333-185398), initially filed by the Partnership on December 12, 2012 (the “Registration Statement”).

On March 27, 2013, the Underwriters exercised their option in full with respect to the Additional Units. The Offering closed on April 2, 2013. The Partnership received proceeds (net of underwriting discounts, commissions and offering expenses) from the Offering of approximately $399.9 million, including the exercise of the option to purchase Additional Units. The Partnership also expects to receive an approximate $8.4 million capital contribution from the general partner to maintain the general partner’s 2.0% general partner interest. As described in the Prospectus Supplement, the Partnership intends to use the net proceeds from the Offering for general partnership purposes, including repayment of amounts outstanding under the Partnership’s revolving credit facility.

The Underwriting Agreement contains customary representations, warranties and agreements of the Partnership, and customary conditions to closing, obligations of the parties and termination provisions. The Partnership has agreed to indemnify the Underwriters against certain liabilities, including liabilities under the Act, or to contribute to payments the Underwriters may be required to make because of any of those liabilities.

As more fully described under the caption “Underwriting” in the Prospectus Supplement, certain of the Underwriters have performed commercial banking, investment banking and advisory services for the Partnership and affiliates from time to time for which they have received customary fees and reimbursement of expenses. The Underwriters or their affiliates may, from time to time, engage in transactions with and perform services for the Partnership and its affiliates in the ordinary course of their business for which they may receive customary fees and reimbursement of expenses. Affiliates of all of the Underwriters are lenders under the Partnership’s revolving credit facility and accordingly will receive a portion of the net proceeds from the Offering.

The foregoing description of the Underwriting Agreement is not complete and is qualified in its entirety by reference to the full text of the Underwriting Agreement, which is attached as Exhibit 1.1 to this Current Report on Form 8-K and incorporated in this Item 1.01 by reference. Legal opinions delivered relating to the Common Units are filed as Exhibits 5.1 and 8.1 to this Current Report on Form 8-K.

Item 9.01 Financial Statements and Exhibits

(d) Exhibits

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 thereunto duly authorized.

Dated: April 2, 2013

INDEX TO EXHIBITS