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, 2011,

or

For the transition period from                      to                     

Commission file number 001-33812

MSCI INC.

(Exact Name of Registrant as Specified in Its Charter)

One Chase Manhattan Plaza, 44^th Floor

New York, New York 10005

(Address of Principal Executive Offices, zip code)

(212) 804-3900

(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  x    NO  ¨

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 Web site, 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.  x

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 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 Act).

YES  ¨    NO  x

The aggregate market value of Common Stock held by non-affiliates of the registrant as of the last business day of the registrant’s most recently completed second fiscal quarter (based on the closing price of these securities as reported by The New York Stock Exchange on June 30, 2011) was approximately $4,461,163,352. Shares of Common Stock held by executive officers and directors of the registrant are not included in the computation. However, the registrant has made no determination that such individuals are “affiliates” within the meaning of Rule 405 under the Securities Act of 1933.

As of February 23, 2012, there were 121,457,571 shares of the Registrant’s Class A common stock, $0.01 par value, outstanding and no shares of Registrant’s Class B common stock, $0.01 par value, outstanding.

Documents incorporated by reference: Portions of the Registrant’s proxy statement for its annual meeting of stockholders, to be held on May 2, 2012, are incorporated herein by reference into Part III of this Form 10-K.

EXPLANATORY NOTE

MSCI Inc. is filing this Amendment No. 1 on Form 10-K/A (the “Amendment”) to our Annual Report on Form 10-K for the year ended December 31, 2011, which was originally filed with the Securities and Exchange Commission (“SEC”) on February 29, 2012 (“original Form 10-K”). The sole purpose of this Amendment is to re-file an amended Exhibit 10.63 in response to comments received from the SEC on a confidential treatment request we made for certain portions of Exhibit 10.63 filed with our original Form 10-K.

No revisions are being made to our financial statements, and other than changes to references in Part IV, Item 15 and the Exhibit Index, and the filing of Exhibit 10.63 and the currently dated 302 certifications, no other modifications or changes have been made to our original Form 10-K or the exhibits filed therewith. As such, this Amendment should be read in conjunction with the original Form 10-K. The information contained in this Amendment does not reflect events occurring subsequent to the filing of our original Form 10-K.

Part IV

(a)(3) Exhibits

The information required by this Item is set forth on the attached Exhibit Index.

SIGNATURES

Pursuant to the requirements of Section 13 or 15(d) 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, on this 20^th day of July, 2012.

EXHIBIT INDEX