Item 1(a)

Name of Issuer:

Ironwood Pharmaceuticals, Inc.

Item 1(b)

Address of Issuer's Principal Executive Offices:

301 Binney Street, Cambridge, Massachusetts 02142

Item 2(a)

Name of Person Filing:

This statement is filed by: (i) Point72 Asset Management, L.P. (“Point72 Asset Management”) with respect to shares of Class A common stock, par value $0.001 per share (“Shares”), of the Issuer held by certain investment funds it manages; (ii) Point72 Capital Advisors, Inc. (“Point72 Capital Advisors Inc.”) with respect to Shares held by certain investment funds managed by Point72 Asset Management; (iii) Cubist Systematic Strategies, LLC (“Cubist Systematic Strategies”) with respect to Shares held by certain investment funds it manages; and (iv) Steven A. Cohen with respect to Shares beneficially owned by Point72 Asset Management, Point72 Capital Advisors Inc., and Cubist Systematic Strategies.

Point72 Asset Management, Point72 Capital Advisors Inc., Cubist Systematic Strategies, and Steven A. Cohen have entered into a Joint Filing Agreement, a copy of which is filed with this Schedule 13G as Exhibit 99.1, pursuant to which they have agreed to file this Schedule 13G jointly in accordance with the provisions of Rule 13d-1(k) of the Act.

Item 2(b)

Address or Principal Business Office:

The address of the principal business office of (i) Point72 Asset Management, Point72 Capital Advisors Inc. and Mr. Cohen is 72 Cummings Point Road, Stamford, CT 06902; and (ii) Cubist Systematic Strategies is 330 Madison Avenue, New York, NY 10173.

Item 2(c)

Citizenship:

Point72 Asset Management is a Delaware limited partnership.  Point72 Capital Advisors Inc. is a Delaware corporation.  Cubist Systematic Strategies is a Delaware limited liability company.  Mr. Cohen is a United States citizen.

Item 2(d)

Title of Class of Securities:

Class A Common Stock, par value $0.001 per share

Item 2(e)

CUSIP Number:

46333X108

Item 3

Not Applicable

Item 4

Ownership:

The percentages used herein are calculated based upon the Shares of Class A common stock issued and outstanding as of November 3, 2016, as reported on the Issuer's quarterly report on Form 10-Q filed with the Securities and Exchange Commission by the Issuer for the quarterly period ended September 30, 2016.

1.  Point72 Asset Management, L.P.

(a) Amount beneficially owned: 1,116,532

(b) Percent of class: 0.9%

(c)(i) Sole power to vote or direct the vote: -0-

(ii) Shared power to vote or direct the vote: 1,116,532

(iii) Sole power to dispose or direct the disposition: -0-

(iv) Shared power to dispose or direct the disposition: 1,116,532

2.  Point72 Capital Advisors, Inc.

(a) Amount beneficially owned: 1,116,532

(b) Percent of class: 0.9%

(c)(i) Sole power to vote or direct the vote: -0-

(ii) Shared power to vote or direct the vote: 1,116,532

(iii) Sole power to dispose or direct the disposition: -0-

(iv) Shared power to dispose or direct the disposition: 1,116,532

3.  Cubist Systematic Strategies, LLC

(a) Amount beneficially owned: 7,919

(b) Percent of class: less than 0.1%

(c)(i) Sole power to vote or direct the vote: -0-

(ii) Shared power to vote or direct the vote: 7,919

(iii) Sole power to dispose or direct the disposition: -0-

(iv) Shared power to dispose or direct the disposition: 7,919

4. Steven A. Cohen

(a) Amount beneficially owned: 1,124,451

(b) Percent of class: 0.9%

(c)(i) Sole power to vote or direct the vote: -0-

(ii) Shared power to vote or direct the vote: 1,124,451

(iii) Sole power to dispose or direct the disposition: -0-

(iv) Shared power to dispose or direct the disposition: 1,124,451

Point72 Asset Management, Point72 Capital Advisors Inc., Cubist Systematic Strategies, and Mr. Cohen own directly no Shares.  Pursuant to an investment management agreement, Point72 Asset Management maintains investment and voting power with respect to the securities held by certain investment funds it manages.  Point72 Capital Advisors Inc. is the general partner of Point72 Asset Management.  Pursuant to an investment management agreement, Cubist Systematic Strategies maintains investment and voting power with respect to the securities held by certain investment funds it manages.  Mr. Cohen controls each of Point72 Capital Advisors Inc. and Cubist Systematic Strategies.  As of December 31, 2016, by reason of the provisions of Rule 13d-3 of the Securities Exchange Act of 1934, as amended, each of (i) Point72 Asset Management, Point72 Capital Advisors Inc., and Mr. Cohen may be deemed to beneficially own 1,116,532 Shares (constituting approximately 0.9% of the Shares outstanding); and (ii) Cubist Systematic Strategies and Mr. Cohen may be deemed to beneficially own 7,919 Shares (constituting less than 0.1% of the Shares outstanding).  Each of Point72 Asset Management, Point72 Capital Advisors Inc., Cubist Systematic Strategies, and Mr. Cohen disclaims beneficial ownership of any of the securities covered by this statement.

Item 5

Ownership of Five Percent or Less of a Class:

If this statement is being filed to report the fact that as of the date hereof the reporting person has ceased to be the beneficial owner of more than five percent of the class of securities, check the following. ☒

Item 6

Ownership of More than Five Percent on Behalf of Another Person:

Not Applicable

Item 7

Identification and Classification of the Subsidiary Which Acquired the Security Being Reported on By the Parent Holding Company:

Not Applicable

Item 8

Identification and Classification of Members of the Group:

Not Applicable

Item 9

Notice of Dissolution of Group:

Not Applicable

Item 10

Certification: