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): August 28, 2015

Crown Castle International Corp.

(Exact Name of Registrant as Specified in its Charter)

1220 Augusta Drive

Suite 600

Houston, TX 77057

(Address of Principal Executive Office)

Registrant’s telephone number, including area code: (713) 570-3000

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 1.01 — ENTRY INTO A MATERIAL DEFINITIVE AGREEMENT

On August 28, 2015, Crown Castle International Corp. (“Company”) entered into sales agreements (“Sales Agreements”) with each of Merrill Lynch, Pierce, Fenner & Smith Incorporated, Barclays Capital Inc., Citigroup Global Markets Inc., Credit Agricole Securities (USA) Inc., J.P. Morgan Securities LLC, Mizuho Securities USA Inc., Mitsubishi UFJ Securities (USA), Inc., Morgan Stanley & Co. LLC, RBC Capital Markets, LLC, SMBC Nikko Securities America, Inc., SunTrust Robinson Humphrey, Inc. and Wells Fargo Securities, LLC (each, a “Sales Agent”), pursuant to which the Company may issue and sell from time to time shares (“Shares”) of its common stock, par value $0.01 per share (“Common Stock”), having an aggregate gross sales price of up to $500,000,000.

Subject to the terms and conditions of the Sales Agreements, the Sales Agents will use their commercially reasonable efforts to sell, on the Company’s behalf, the Shares that may be offered by the Company from time to time under the Sales Agreements. The sales, if any, of the Shares made under the Sales Agreements will be made by means of ordinary brokers’ transactions on the New York Stock Exchange or otherwise at market prices prevailing at the time of sale, at prices related to prevailing market prices or, subject to specific instructions of the Company, at negotiated prices. Under the terms of the Sales Agreements, the Company may also sell Shares to any Sales Agent as principal for its own account. If the Company sells Shares to any Sales Agent as principal, it will enter into a separate terms agreement setting forth the terms of such transaction with the applicable Sales Agent. Actual sales will depend on a variety of factors to be determined by the Company from time to time.

The Shares will be sold pursuant to the Company’s shelf registration statement on Form S-3 (Registration No. 333-203074). The above description of the Sales Agreements does not purport to be a complete statement of the parties’ rights and obligations under the Sales Agreements and is qualified in its entirety by reference to the terms of the Form of Sales Agreement. For a complete description of the terms and conditions of the Sales Agreements, please refer to the Form of Sales Agreement, which is incorporated herein by reference and attached hereto as Exhibit 1.1.

ITEM 9.01 — FINANCIAL STATEMENTS AND EXHIBITS

(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.

Date: August 28, 2015

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