UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

FORM 10-K/A

Amendment No. 1

(Mark One)

For the fiscal year ended December 31, 2015

OR

For the transition period from                 to                

Commission File Number: 001-36131

Endurance International Group Holdings, Inc.

(Exact name of registrant as specified in its charter)

(781) 852-3200

(Registrant’s telephone number, including area code)

Securities registered pursuant to Section 12(b) of the Act:

Securities registered pursuant to Section 12(g) of the Act:

None

Indicate by check mark if the registrant is a well-known seasoned issuer, as defined in Rule 405 of the Securities Act.    Yes  ¨    No  x

Indicate by check mark if the registrant is not required to file reports pursuant to Section 13 or Section 15(d) of the Act.    Yes  ¨    No  x

Indicate by check mark whether the registrant (1) has filed all reports required to be filed by Section 13 or 15(d) of the Securities Exchange Act of 1934 during the preceding 12 months (or for such shorter period that the registrant was required to file such reports), and (2) has been subject to such filing requirements for the past 90 days.    Yes  x    No  ¨

Indicate by check mark whether the registrant has submitted electronically and posted on its corporate Website, if any, every Interactive Data File required to be submitted and posted pursuant to Rule 405 of Regulation S-T (§232.405 of this chapter) during the preceding 12 months (or for such shorter period that the registrant was required to submit and post such files).    Yes  x    No  ¨

Indicate by check mark if disclosure of delinquent filers pursuant to Item 405 of Regulation S-K (§229.405 of this chapter) is not contained herein, and will not be contained, to the best of registrant’s knowledge, in definitive proxy or information statements incorporated by reference in Part III of this Form 10-K or any amendment to this Form 10-K.    ¨

Indicate by check mark whether the registrant is a large accelerated filer, an accelerated filer, a non-accelerated filer, or a smaller reporting company. See the definitions of “large accelerated filer,” “accelerated filer” and “smaller reporting company” in Rule 12b-2 of the Exchange Act. (Check one)

Indicate by check mark whether the registrant is a shell company (as defined in Rule 12b-2 of the Exchange Act).    Yes  ¨    No  x

The aggregate market value of common stock held by non-affiliates of the registrant based on the closing price of the registrant’s common stock as reported on the NASDAQ Global Select Market on June 30, 2015, was $1,250,205,625.

As of June 28, 2016 there were 140,897,191 shares of the registrant’s common stock, $0.0001 par value per share, outstanding.

DOCUMENTS INCORPORATED BY REFERENCE

Portions of the registrant’s definitive proxy statement for its 2016 Annual Meeting of Stockholders, which was filed pursuant to Regulation 14A with the Securities and Exchange Commission on April 12, 2016, are incorporated by reference into Part III of this Annual Report on Form 10-K.

EXPLANATORY NOTE

This Amendment No. 1 to Form 10-K (this “Amendment”) amends the Annual Report on Form 10-K for the fiscal year ended December 31, 2015, originally filed on February 29, 2016 (the “Original 10-K”), of Endurance International Group Holdings, Inc. (the “Company”). The Company is filing this Amendment to include in Item 15 the separate financial statements of WZ (UK) Limited (“WZ UK”) as required by Rule 3-09 of Regulation S-X, which were not included in the Original 10-K and are due within six months of December 31, 2015.

As of December 31, 2015, the Company held a 49% interest in WZ UK and WZ UK met the significance test of an equity investee for the Company’s fiscal year ended December 31, 2015. In accordance with Rule 3-09 of Regulation S-X, the WZ UK financial statements as of and for the year ended December 31, 2015 were audited in accordance with United Kingdom Accounting Standards (United Kingdom Generally Accepted Accounting Practice). The WZ UK financial statements as of and for the period ended December 31, 2014 are unaudited.

This Amendment should be read in conjunction with the Original 10-K and the Company’s other filings made with the Securities and Exchange Commission subsequent to the filing of the Original 10-K on February 29, 2016. The Original 10-K has not been amended or updated to reflect events occurring after February 29, 2016, except as specifically set forth in this Amendment.

For a list of the consolidated financial statements included herein, which are incorporated into this Item by reference, see Index to Consolidated Financial Statements on page 90 of the Original 10-K.

The financial statements of WZ UK are provided as Exhibit 99.1 to this Amendment.

The exhibits listed in the Exhibit Index of the Original 10-K and this Amendment are filed with, or incorporated by reference in, this report.

SIGNATURE

Pursuant to the requirements of Section 13 or 15(d) of the Securities Exchange Act of 1934, the registrant has duly caused this Amendment to be signed on its behalf by the undersigned, thereunto duly authorized.

EXHIBIT INDEX