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): April 3, 2013

ERP OPERATING LIMITED PARTNERSHIP

(Exact Name of Registrant as Specified in its Charter)

Registrant’s telephone number, including area code: (312) 474-1300

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 (see General Instruction A.2. below):

Item 8.01. Other Events.

On April 3, 2013, ERP Operating Limited Partnership, an Illinois limited partnership (the “Company”), agreed to issue $500,000,000 aggregate principal amount of 3.00% Notes due April 15, 2023 (the “Notes”) in a public offering. The Company agreed to sell the Notes pursuant to a Terms Agreement, dated as of April 3, 2013, among the Company and each of Citigroup Global Markets Inc., Barclays Capital Inc., Morgan Stanley & Co. LLC and RBC Capital Markets, LLC, as representatives of the underwriters named therein. The Notes will be issued pursuant to an Indenture, dated as of October 1, 1994, between the Company and The Bank of New York Mellon Trust Company, N.A. (formerly known as The Bank of New York Trust Company, N.A.) (as successor to J.P. Morgan Trust Company, National Association, as successor to Bank One Trust Company, N.A., successor to The First National Bank of Chicago) (the “Trustee”), as supplemented by the First Supplemental Indenture, dated as of September 9, 2004, by and between the Company and the Trustee, as further supplemented by the Second Supplemental Indenture, dated as of August 23, 2006, by and between the Company and the Trustee, as further supplemented by the Third Supplemental Indenture, dated as of June 4, 2007, by and between the Company and the Trustee, and as further supplemented by the Fourth Supplemental Indenture, dated as of December 12, 2011, by and between the Company and the Trustee.

Item 9.01. Financial Statements and Exhibits.

2

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