This proxy statement is furnished in connection with the solicitation of proxies by the board of directors, or the Board, of Sesen Bio, Inc., alternatively referred to herein as “Sesen,” the “Company,” “we” or “us” for use at the 2019 Annual Meeting of Stockholders, or the 2019 Annual Meeting, on Wednesday, June 19, 2019, beginning at 8:00 a.m., Eastern Daylight Time, at 1735 Market Street, Suite 2300, Philadelphia, PA 19103. On April 26, 2019, the record date for the determination of stockholders entitled to vote at the 2019 Annual Meeting, or the Record Date, there were outstanding and entitled to vote an aggregate of 77,464,782 shares of our common stock, par value $0.001 per share, or common stock. Each share of common stock entitles the record holder thereof to one vote on each of the matters to be voted on at the 2019 Annual Meeting.

If you are the “record holder” of your shares, meaning that you own your shares in your own name and not through a bank, broker or other nominee, you may vote in one of four ways:

If the shares you own are held in “street name” by a bank, broker or other nominee record holder, which, for convenience, we collectively refer to in this proxy statement as a brokerage firm, your brokerage firm, as the record holder of your shares, is required to vote your shares according to your instructions. In order to vote your shares, change your vote or revoke your instructions, you will need to follow the directions your brokerage firm provides you. Many brokerage firms also offer the option of providing for voting over the Internet or by telephone, instructions for which, if available, would be provided by your brokerage firm on the voting instruction form that it delivers to you. Because most brokerage firms are member organizations of the New York Stock Exchange, or NYSE, the rules of the NYSE will likely govern how your brokerage firm would be permitted to vote your shares in the absence of instruction from you. Under the current rules of the NYSE, if you do not give instructions to your brokerage firm, it will still be able to vote your shares with respect to certain “discretionary” items, but will not be allowed to vote your shares with respect to certain “non-discretionary” items. The approval of an amendment to our Restated Certificate of Incorporation to effect a reverse stock split of our common stock (Proposal 2) and the ratification of Ernst & Young LLP as our independent registered public accounting firm (Proposal 4) are each3) is considered to be a discretionary itemsitem under the NYSE rules, and your brokerage firm will be able to vote on these itemsthis item even if it does not receive instructions from you, so long as it holds your shares in its name. The election of class II directors (Proposal 1) and the approval of an amendment to our 2014 Plan (Proposal 3)2) are each “non-discretionary” items, meaning that if you do not instruct your brokerage firm on how to vote with respect to Proposal 1 or Proposal 3,2, your brokerage firm will not vote with respect to those proposals and your shares will be counted as “broker non-votes.” “Broker non-votes” are shares that are held in “street name” by a brokerage firm that indicates on its proxy that it does not have or did not exercise discretionary authority to vote on a particular matter.

The holders of 38,732,392 shares of our common stock, representing a majority of the shares of our common stock issued and outstanding and entitled to vote at the 2019 Annual Meeting, will constitute a quorum for the transaction of business at the 2019 Annual Meeting. Shares of common stock represented in person or by proxy (including shares which abstain or do not vote with respect to one or more of the matters presented for stockholder approval) will be counted for purposes of determining whether a quorum is present at the 2019 Annual Meeting.

The following votes are required for approval of the proposals being presented at the 2019 Annual Meeting:

Broker non-votes and abstentions with respect to a proposal are counted as present or represented by proxy for purposes of establishing a quorum. If a quorum is present, votes to withhold will have no effect on the outcome of the votes on Proposal 1 (election

(election of directors), but abstentions will count as votes against Proposal 2 (approval of an amendment to our Certificate of Incorporation to effect a reverse stock split), Proposal 3 (approval of an amendment to our 2014 Plan) and Proposal 43 (ratification of Ernst & Young LLP as our independent registered public accounting firm for the current fiscal year). Broker non-votes will have no effect on the outcomes of Proposal 1, Proposal 2 Proposal 3 or Proposal 4.3.

All outstanding shares of our common stock represented by valid and unrevoked proxies received in time for the 2019 Annual Meeting will be voted. With respect to the election of directors, a stockholder may (i) vote for the election of all of the named director nominees, (ii) withhold authority to vote for all such director nominees, or (iii) vote for the election of one or more director nominees and withhold authority to vote for one or more nominee directors.

The proposals (x) to approve an amendment to the our Restated Certificate of Incorporation to effect a reverse stock split of our common stock at a ratio in the range of 1:3 to 1:10, such ratio to be determined in the discretion of our Board, (y) to approve an amendment to our 2014 Plan to (i) increase the number of shares reserved for issuance under our 2014 Plan by 7,908,972 shares and (ii) eliminate the "evergreen" or automatic replenishment provision of the 2014 Plan pursuant to which the number of shares authorized for issuance under the 2014 Plan is automatically increased on an annual basis, and (z)(y) to ratify Ernst & Young LLP as our independent registered public accounting firm for the fiscal year ending December 31, 2019 each allow stockholders to (i) vote “FOR” the matter, (ii) vote “AGAINST” the matter, or (iii) “ABSTAIN” from voting on the matter.

Shares will be voted on each proposal as instructed in the accompanying proxy. However, if no instructions are given on a validly signed and returned proxy (other than with respect to broker non-votes), the shares will be voted in accordance with the Company’s recommendations as follows: (i) “FOR” the election of the named director nominees; (ii) “FOR” approval of the amendment to the our Restated Certificate of Incorporation to effect a reverse stock split of our common stock at a ratio in the range of 1:3 to 1:10, such ratio to be determined in the discretion of our Board; (iii) “FOR” approval of an amendment to our 2014 Plan to (A) increase the number of shares reserved for issuance under our 2014 Plan by 7,908,972 shares and (B) eliminate the "evergreen" or automatic replenishment provision of the 2014 Plan pursuant to which the number of shares authorized for issuance under the 2014 Plan is automatically increased on an annual basis; and (iv)(iii) “FOR” ratification of Ernst & Young LLP as our independent registered public accounting firm for the fiscal year ending December 31, 2019.

We intend to first make available this proxy statement, the enclosed proxy card and our Annual Report on Form 10-K for the year ended December 31, 2018, or the 2018 Annual Report, on or about May 6, 2019.

The following table sets forth the name and age of each of our directors as of May 6, 2019.

Our Board is currently authorized to have six members and currently consists of six members. Our Board is divided into three classes, with members of each class serving staggered three-year terms. The members of the classes are divided as follows:

Our Board observes all applicable criteria for independence established by the rules of the Nasdaq Stock Market, or the Nasdaq Stock Market Rules and other governing laws and applicable regulations, including the requirement that a majority of our directors be independent as defined under the Nasdaq Stock Market Rules. Under Rule 5605(a)(2) of the Nasdaq Stock Market Rules, a director will only qualify as an “independent director” if, in the opinion of our Board, that person does not have a relationship that would interfere with the exercise of independent judgment in carrying out the responsibilities of a director. Our Board has determined that all of our directors with the exceptions of Mr. Dan and Dr. Cannell are independent as defined under the Nasdaq Stock Market Rules.

The positions of chair of the Board and chief executive officer are presently separated at our company. The duties of the chair of the Board include the following:

Our Board has established an audit committee, a compensation committee, and a nominating and corporate governance committee, each of which operates under a charter that has been approved by our Board. Copies of the committee charters are posted on the Investor Relations section of our website, which is located at www.sesenbio.com.

The members of our audit committee are Ms. Henderson, Dr. Dixon and Mr. Lynch. Ms. Henderson chairs our audit committee. Our audit committee’s responsibilities include:

The members of our compensation committee are Dr. Dixon, Ms. Henderson and Mr. Lynch. Dr. Dixon chairs our compensation committee. Our compensation committee’s responsibilities include:

The members of our nominating and corporate governance committee are Mr. Lynch and Ms. Henderson. Mr. Lynch chairs our nominating and corporate governance committee. Our nominating and corporate governance committee’s responsibilities include:

Our Board met five times during 2018. During 2018, each director attended at least 75% of the aggregate of the number of Board meetings and the number of meetings held by all committees of the Board on which he or she then served.

Our Board oversees our risk management processes directly and through its committees. Our management is responsible for risk management on a day-to-day basis. The role of our Board and its committees is to oversee the risk management activities of management. They fulfill this duty by discussing with management the policies and practices utilized by management in assessing and managing risks and providing input on those policies and practices. In general, our Board oversees risk management activities relating to business strategy, acquisitions, capital allocation, organizational structure and certain operational risks; our audit committee oversees risk management activities related to financial controls and legal and compliance risks; our nominating and corporate governance committee oversees risk management activities relating to Board composition and management succession planning; and our compensation committee oversees risk management activities relating to our compensation policies and practices. Each committee reports to the full Board on a regular basis, including reports with respect to the committee’s risk oversight activities as appropriate. In addition, since risk issues often overlap, committees from time to time request that the full Board discusses particular risks.

The process followed by our nominating and corporate governance committee and our Board to identify and evaluate director candidates may include requests to Board members and others for recommendations, evaluation of the performance on our Board and its committees of any existing directors being considered for nomination, consideration of biographical information and background material relating to potential candidates and, particularly in the case of potential candidates who are not then serving on our Board, interviews of selected candidates by members of our nominating and corporate governance committee and our Board.

Our management will give appropriate attention to written communications that are submitted by stockholders, and will respond if and as appropriate. Stockholders may communicate with our management by writing to our Corporate Secretary at Sesen Bio, Inc., Attention: Corporate Secretary, or by calling (617) 444-8550. Additional information about contacting our Board or management is available on the Investor Relations section of our website, which is located at www.sesenbio.com.

Our executive compensation program is administered by the compensation committee of our Board, subject to the oversight and approval of our full Board. Our compensation committee reviews our executive compensation practices on an annual basis and based on this review makes recommendations to our Board for approval, which has full discretion to approve or modify the recommendations of the compensation committee.

Our Board has adopted corporate governance guidelines to assist in the exercise of its duties and responsibilities and to serve the best interests of our stockholders. The guidelines provide that:

Our Board has adopted written policies and procedures for the review of any transaction, arrangement or relationship in which our company is a participant, the amount involved exceeds $120,000 and one of our executive officers, directors, director nominees or 5% stockholders, or their immediate family members, each of whom we refer to as a “related person,” has a direct or indirect material interest. As a smaller reporting company, we are also required to review and approve any transaction, arrangement or relationship in which our company is a participant, the amount involved exceeds the lesser of $120,000 and or one percent of the average of our total assets at year-end for the last two completed fiscal years, and a related person has a direct or indirect material interest. Because one percent of our average total assets for the past two fiscal years has exceeded $120,000, our Board has continued to apply the $120,000 threshold under our policy.

Since January 1, 2017, we have engaged in the following transactions with our directors, executive officers, holders of more than 5% of our voting securities, and affiliates or immediate family members of our directors, executive officers, and holders of more than 5% of our voting securities. We believe that all of these transactions were on terms as favorable as could have been obtained from unrelated third parties.

The following table sets forth information regarding compensation awarded to, earned by or paid to each of our named executive officers for the years ended December 31, 2018 and 2017.

The following table sets forth information regarding all outstanding stock options held by each of our named executive officers as of December 31, 2018.

Pursuant to their respective employment agreements with the Company, upon execution and effectiveness of a release of claims, each of Dr. Cannell, Mr. Fitzgerald and Dr. Kim will be entitled to severance payments if his employment is terminated under specified circumstances.

We maintain a defined contribution 401(k) retirement plan for our employees in which substantially all of our full-time U.S. employees are eligible to participate, including our named executive officers. Participants may contribute a percentage of their annual compensation to this plan, subject to statutory limitations. We made matching contributions of $18,281 to the plan for the year ended December 31, 2018.

Our certificate of incorporation limits the personal liability of directors for breach of fiduciary duty to the maximum extent permitted by the Delaware General Corporation Law, or the DGCL, and provides that no director will have personal liability to

us or to our stockholders for monetary damages for breach of fiduciary duty as a director. However, these provisions do not eliminate or limit the liability of any of our directors:

The audit committee of the Board of Sesen Bio, Inc. has reviewed Sesen’s audited financial statements for the fiscal year ended December 31, 2018 and discussed them with Sesen’s management and Ernst & Young LLP, Sesen’s independent registered public accounting firm.

Ernst & Young LLP provided audit and tax services to us consisting of the audit of our 2018 and 2017 financial statements and tax compliance services. The following table summarizes the fees for Ernst & Young LLP services to us for the last two fiscal years.

In accordance with the terms of our Restated Certificate of Incorporation and our Amended and Restated By-laws, our Board is divided into three classes, with each class serving staggered three-year terms. Upon the expiration of the term of a class of directors, directors in that class will be eligible to be elected for a new three-year term at the annual meeting of stockholders in the year in which their term expires. The members of the classes are divided as follows:

The audit committee of our Board has selected the firm of Ernst & Young LLP as our independent registered public accounting firm2015 and continuing for each fiscal year until, and including, the fiscal year ending December 31, 2019. Ernst & Young LLP2024, the number of shares authorized for issuance under the 2014 Plan is increased to the least of (i) 1,102,362 shares of of our common stock, (ii) four percent (4%) of the number of outstanding shares of our common stock on such date and (iii) an amount determined by our Board.

Based solely on our review of copies of reports filed by individuals and entities required to make filings pursuant to Section 16(a) of the Exchange Act or written representations from such individuals or entities, we believe that during 2018 all filings required to be made by such individuals or entities were timely made in accordance with the Exchange Act.

Some banks, brokers and other nominee record holders may be participating in the practice of “householding” notices of internet availability, proxy statements and annual reports. This means that only one copy of our Notice, or proxy statement, and annual report may have been sent to multiple stockholders in the same household. We will promptly deliver a separate copy of either document to any stockholder upon request submitted in writing to us at Sesen Bio, Inc., 245 First Street, Suite 1800 Cambridge, MA 02142, Attention: Corporate Secretary, or by calling (617) 444-8550. Any stockholder who wants to receive separate copies of the Notice, proxy statement or annual report in the future, or who is currently receiving multiple copies and would like to receive only one copy for his or her household, should contact his or her bank, broker or other nominee record holder, or contact us at the above address or phone number.

Proposals of stockholders intended to be presented at our annual meeting of stockholders to be held in 2020, or the 2020 Annual Meeting, pursuant to Rule 14a-8 promulgated under the Exchange Act must be received by us at our principal offices, 245 First Street, Suite 1800 Cambridge, MA 02142, Attention: Corporate Secretary, no later than January 7, 2020, the date that is 120 days prior to the first anniversary of the date of this proxy statement, in order to be included in the proxy statement and proxy card relating to that meeting.