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): June 11, 2013

Infinity Pharmaceuticals, Inc.

(Exact name of registrant as specified in charter)

Registrant’s telephone number, including area code: (617) 453-1000

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:

(e) At our Annual Meeting of stockholders held on June 11, 2013 (the “Annual Meeting”), our stockholders approved Amendment No. 4 to our 2010 Stock Incentive Plan, as amended (the “2010 Plan”), which amendment had previously been adopted by our Board of Directors, subject to shareholder approval, to increase the number of shares of our common stock that may be issued pursuant to awards granted under the 2010 Plan by an additional 1,485,000 shares. The description of the 2010 Plan set forth herein does not purport to be complete and is qualified in its entirety by reference to the full text of the 2010 Plan attached hereto as Exhibit 10.1 and is incorporated herein by reference.

Key features of the 2010 Plan are summarized as follows:

Number of Shares Available for Award

Up to 7,485,000 shares of our common stock may be issued pursuant to awards granted under the 2010 Plan, subject to adjustment in the event of stock splits and other similar events. Shares issued under the 2010 Plan may be authorized and unissued shares, or may be issued from shares that we have reacquired (provided that open-market purchases of shares using the proceeds from the exercise of awards do not increase the number of shares available for future grants).

The 2010 Plan uses a “fungible share” concept under which the awards of options and stock appreciation rights, or SARs, cause one share per covered share to be removed from the available share pool, while the award of restricted stock, restricted stock units, or other stock-based awards where the price charged for the award is less than 100% of the fair market value of our common stock will be counted against the pool as 1.35 shares. Shares covered by awards under the 2010 Plan that are forfeited, cancelled or otherwise expire without having been exercised or settled, or that are settled by cash or other non-share consideration, become available for issuance pursuant to a new award and will be credited back to the pool at the same rates described above. Shares that are tendered or withheld to pay the exercise price of an award or to satisfy tax withholding obligations are not available for issuance pursuant to new awards. Shares are subtracted for exercises of SARs using the proportion of the total SAR that is exercised, rather than the number of shares actually issued.

Types of Awards

The 2010 Plan provides for the grant of incentive stock options intended to qualify under Section 422 of the Internal Revenue Code of 1986, as amended, or “Code”, nonstatutory stock options, SARs, restricted stock, restricted stock units and other stock-based and cash-based awards as described below.

Incentive Stock Options and Nonstatutory Stock Options. Optionees receive the right to purchase a specified number of shares of Common Stock at a specified option price and subject to such other terms and conditions as are specified in connection with the option grant. Subject to the limitations described below, options must be granted at an exercise price equal to or greater than the fair market value of our common stock on the date of grant. Under the 2010 Plan, options may not be granted for a term in excess of ten years. Options may not provide for the automatic grant of additional shares in connection with the exercise of the original option, and options may not provide for the payment or accrual of dividend equivalents. The 2010 Plan permits the following forms of payment of the exercise price of options:

Director Options. The 2010 Plan provides for the automatic grant of options to members of our board of directors who are not employees as follows:

Each automatic grant will have an exercise price per share equal to the fair market value per share of our common stock on the grant date and will have a term of 10 years, subject to earlier termination following the optionee’s cessation of board service unless otherwise modified by our board of directors. In addition, these options will immediately vest in full upon certain changes in control or ownership or upon the optionee’s death or disability while a board member. Our board of directors retains the specific authority to increase or decrease the number of shares subject to options granted to non-employee directors under the 2010 Plan.

Stock Appreciation Rights. A SAR is an award entitling the holder, upon exercise, to receive an amount in common stock, cash, or a combination thereof determined by reference to appreciation, from and after the date of grant, in the fair market value of a share of our common stock. SARs may be granted independently or in tandem with a stock option.

Restricted Stock Awards. Restricted stock awards entitle recipients to acquire shares of our common stock, subject to the right of the company to repurchase some or all of those shares from the recipient in the event that the conditions specified in the applicable award are not satisfied prior to the end of the restriction period established for that award. The right to receive any dividends with respect to restricted stock will be conditioned on the vesting of the award.

Restricted Stock Unit Awards. Restricted stock unit awards, or RSUs, entitle the recipient to receive shares of our common stock to be delivered at the time the shares vest pursuant to the terms and conditions established by our board of directors. The right to receive any dividend equivalents with respect to RSUs, if such rights are provided, will be conditioned on the vesting of the award.

Other Stock-Based Awards. Under the 2010 Plan, our board of directors has the right to grant other awards based upon our common stock, with these awards having terms and conditions determined by the board. These awards may be paid in shares of our common stock or in cash.

Performance Conditions. The Compensation Committee of our board of directors, referred to herein as the Compensation Committee, may determine, at the time of grant, that a restricted stock award, RSU or other stock-based award granted to an officer will vest solely upon the achievement of specified performance criteria designed to qualify for deduction under Section 162(m) of the Code. The performance criteria for each such award will be based on one or more of the following measures:

Such performance goals may be adjusted to exclude any one or more of:

These performance goals may vary by participant and be different for different awards, be particular to a participant or department, business, subsidiary or other unit in which the participant works, and may cover any period specified by the Compensation Committee. These goals will, however, be set by the Compensation Committee within the time period prescribed by, and will otherwise comply with the requirements of, Section 162(m) of the Code.

Transferability of Awards

In general, awards may not be sold, assigned, transferred, pledged or otherwise encumbered by the person to whom they are granted, either voluntarily or by operation of law, except by will or the laws of descent and distribution or, other than in the case of an incentive stock option, pursuant to a qualified domestic relations order. During the life of the participant, awards are exercisable only by the participant. With the consent of our board of directors, a participant can transfer an award without payment to an immediate family member, family trust, or certain other related entities to the extent the rules under Form S-8 would cover the transferee.

Eligibility to Receive Awards

Employees, officers, directors, consultants and advisors of our company and its subsidiaries, and of other business ventures in which we have a controlling interest, are eligible to be granted awards under the 2010 Plan. Under present law, however, incentive stock options may only be granted to employees of Infinity Pharmaceuticals, Inc. and its subsidiaries.

The maximum number of shares with respect to which awards may be granted to any participant under the 2010 Plan may not exceed 1,000,000 shares per calendar. For purposes of this limit, the combination of a stock option in tandem with SAR is treated as a single award.

Administration

The 2010 Plan is administered by our board of directors, which has the authority to adopt, amend and repeal the administrative rules, guidelines and practices relating to the 2010 Plan and to interpret the provisions of the 2010 Plan. Pursuant to the terms of the 2010 Plan, our board of directors may delegate authority under the 2010 Plan to one or more committees or subcommittees of the board, and it has authorized the Compensation Committee to administer certain aspects of the 2010 Plan, including the granting of options to executive officers. In addition, our board of directors may authorize one or more of our officers to grant stock options to eligible participants other than our executive officers, subject to limitations set by the Compensation Committee. Our Chief Executive Officer and President, Adelene Q. Perkins, has been authorized to grant such stock options.

Subject to any applicable limitations contained in the 2010 Plan, our board of directors, the Compensation Committee, or any other committee to which the board delegates authority, as the case may be, selects the recipients of awards and determines:

Our board of directors is required to make appropriate adjustments in connection with the 2010 Plan and any outstanding awards to reflect stock splits, stock dividends, recapitalizations, spin-offs and other similar changes in capitalization. The 2010 Plan also contains provisions addressing the consequences of any “Reorganization Event”, which is defined as:

In connection with a Reorganization Event, our board of directors or the Compensation Committee may take any one or more of the following actions as to all or any outstanding Awards on such terms as the board or Compensation Committee determines:

Unless otherwise provided for in the instrument evidencing any stock option, SAR or any other agreement between us and a 2010 Plan participant, effective immediately prior to a “Change in Control Event” (as this term is defined in the 2010 Plan), all stock options and SARs then outstanding will immediately become exercisable in full. Unless otherwise provided in the instrument evidencing a restricted stock award, RSU or any other agreement between us and a 2010 Plan participant, effective immediately prior to a Change in Control Event all restrictions and conditions on all restricted stock awards and RSUs then outstanding will automatically be deemed terminated and/or satisfied. Our board of directors may specify in an award at the time of grant the effect of a Change in Control Event on an other stock-based award. The deferred compensation rules of Section 409A of the Code may delay payment where such delay is required to comply with its rules or may cause, in certain circumstances, RSUs to be terminated without any payment in exchange for such termination.

Except as described above, our board of directors or the Compensation Committee may at any time provide that any award will become immediately exercisable in full or in part, free of some or all restrictions or conditions, or otherwise realizable in full or in part, as the case may be.

If any award expires or is terminated, surrendered, canceled or forfeited, the unused shares of our common stock covered by such award will again be available for grant under the 2010 Plan, subject, in the case of incentive stock options, to any limitations under the Code.

Substitute Awards

In connection with a merger or consolidation of an entity with our company, or the acquisition by our company of property or stock of an entity, our board of directors may grant awards in substitution for any stock options or other stock or stock-based awards granted by such entity or an affiliate thereof. Substitute awards may be granted on such terms as the board deems appropriate in the circumstances, notwithstanding any limitations on options contained in the 2010 Plan. Substitute Options will not count against the 2010 Plan’s overall share limit, except as may be required by the Code.

Restrictions on Repricing

Unless our stockholders approve such action (or it is appropriate under a change in capitalization, a reorganization event, or a Change in Control Event), the 2010 Plan provides that we may not:

Provisions for Foreign Participants

Our board of directors or the Compensation Committee may modify awards granted to participants who are foreign nationals or employed outside the United States, or establish subplans or procedures under the 2010 Plan to recognize differences in laws, rules, regulations or customs of such foreign jurisdictions with respect to tax, securities, currency, employee benefit or other matters.

Amendment or Termination

No Award may be made under the 2010 Plan after May 25, 2020 but awards previously granted may extend beyond that date. Our board of directors may at any time amend, suspend or terminate the 2010 Plan; provided that, to the extent determined by the Board, no amendment requiring stockholder approval under any applicable legal, regulatory or listing requirement will become effective until such stockholder approval is obtained.

At the Annual Meeting, our stockholders voted on three matters as follows:

Additionally, at our Annual Meeting our stockholders approved our 2013 Employee Stock Purchase Plan (the “2013 ESPP”) and the reservation of 250,000 shares for future issuance thereunder, which plan had previously been adopted by our Board of Directors, subject to shareholder approval. A copy of the 2013 ESPP and an amendment thereto is filed as Exhibit 99.1 hereto and is incorporated herein by reference.

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 hereunto duly authorized.