UNITED STATES

SECURITIES AND EXCHANGE COMMISSION

Washington, D.C. 20549

Form 10-Q/A

(Mark One)

For the quarterly period ended September 30, 2011

OR

For the transition period from              to             

Commission file number 001-35186

SPIRIT AIRLINES, INC.

(Exact Name of Registrant as Specified in Its Charter)

(954) 447-7920

(Registrant’s Telephone Number, Including Area Code)

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 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.

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

Indicate the number of shares outstanding of each of the issuer’s classes of common stock, as of the close of business on December 21, 2011:

Explanatory Note

This Amendment No. 1 to Form 10-Q (this “Amendment Filing”) amends the Quarterly Report on Form 10-Q of Spirit Airlines, Inc. (the “Company”) for the quarter ended September 30, 2011, which was originally filed on October 27, 2011 (the “Original Filing”). The Company is filing this Amendment Filing solely for the purpose of re-filing Exhibit 10.1 thereto in response to comments received from the Staff of the Securities and Exchange Commission (the “SEC”) in connection with a confidential treatment request with respect to the Second Amendment to Signatory Agreement, effective as of September 6, 2011, by and between the Company and U.S. Bank National Association (the “U.S. Bank Amendment”). As a result, portions the U.S. Bank Amendment that were omitted in Exhibit 10.1 to the Original Filing, have been provided in Exhibit 10.1 filed herewith.

This Amendment Filing does not modify or update any part of the information set forth in the Original Filing other than Exhibit 10.1.

PART II. OTHER INFORMATION

2

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.

EXHIBIT INDEX