We are providing our proxy materials to record stockholders by sending a printed copy of the full set of our proxy materials, or the Proxy Materials, consisting of the Noticenotice of Annual Meetingannual meeting of Stockholders,stockholders, this Proxy Statementproxy statement and a proxy card or voting information form by mail. As permitted by Securities and Exchange Commission, or the SEC, rules, we are also providing access to the Proxy Materials on the Internet.

ToIn order to attend the 20202021 Annual Meeting, you will need the control number included on your proxy card or voting authorization form. You will need tomust register to attend the 2020 Annual Meeting prior to the deadline of 5:00 p.m., Eastern Daylight Time, on May 4, 2020, or the Registration Deadline. When registering you will be asked to provide the control number located inside the shaded gray box on your proxy card or voting instruction form, or the Control Number, as described in the proxy card or voting instruction form. Uponat www.proxydocs.com/SESN. After completing your registration, you will receive further instructions via email, one hour prior to the start of the 2020 Annual Meeting, including a unique link that will allow youto access to the 20202021 Annual Meeting and to vote and submit questions during the Annual Meeting. You will have the ability to submit questions prior to and during the 20202021 Annual Meeting. As part of the registration process, you must enter the control number located on your proxy card or voting instruction form. If you are a beneficial owner of shares registered in the name of a broker, bank or other nominee, you will also need to provide the registered name on your account and the name of your broker, bank or other nominee as part of the registration process. Please be sure to follow instructions found on your proxy card or voting authorization form and subsequent instructions that will be delivered to you via email.instruction form.

Technical assistance will be available one hour prior to and during the 20202021 Annual Meeting. Information related to technical assistance will be provided in the email with the sign-in instructions. We recommend that you log in at least 15 minutes before the 20202021 Annual Meeting to ensure you are logged in when the 20202021 Annual Meeting starts.

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 three ways:ways prior to the 2021 Annual Meeting:

All proxies that are executed or are otherwise submitted over the Internet or by telephone will be voted on the matters set forth in the accompanying Noticethis proxy statement in accordance with the stockholders’ instructions. However, if no choice is specified on a proxy as to one or more of the proposals, the proxy will be voted in accordance with theour Board's recommendations on such proposals as set forth in this proxy statement.

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 splitincrease the number of shares of our common stock (Proposal 2) and the ratification of Ernst & Young LLP as our independent registered public accounting firm (Proposal 3)5) are each considered to be discretionary items under the NYSE rules, and your brokerage firm will be able to vote on these items even if it does not receive instructions from you, so long as it holds your shares in its name. The election of a Class III directorI directors (Proposal 1), the approval of an amendment to our 2014 Incentive Plan (Proposal 3), the approval of an amendment to our 2014 ESPP (Proposal 4) and the approval of the compensation of our named executive officers (Proposal 4) and the recommendation of the frequency of future advisory votes on the compensation of our named executive officers (Proposal 5)6) are all a “non-discretionary” items, meaning that if you do not instruct your brokerage firm on how to vote with respect to Proposals 1, 3, 4 or 5,6, your brokerage firm will not vote with respect to such proposal 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 54,994,38584,693,114 shares of our common stock, representing a majority in the voting power of the shares of our common stock issued and outstanding and entitled to vote at the 20202021 Annual Meeting, present or represented by proxy, will constitute a quorum for the transaction of business at the 20202021 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 20202021 Annual Meeting. Stockholders who participate the 2021 Annual Meeting via the live webcast will be deemed present for purposes of the quorum requirement.

The following votes are required for approval of the proposals being presented at the 20202021 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 of directors), but abstentions Abstentions will count as votes against Proposal 2 (approval of an amendment to our Restated Certificate of Incorporation to increase the number of authorized shares of common stock from 200,000,000 to 400,000,000), but will no effect a reverseon the outcome of Proposal 3 (approval of an amendment to our 2014 Incentive Plan to increase the number of shares of common stock split)reserved for issuance under the 2014 Incentive Plan by 12,000,000 shares of common stock), Proposal 34 (approval of an amendment to our 2014 ESPP to increase the number of shares of common stock reserved for issuance under the 2014 ESPP by 2,300,000 shares of common stock), Proposal 5 (ratification of Ernst & Young LLP as our independent registered public accounting firm for the current fiscal year), Proposal 4 (Say-on-Pay) and Proposal 5 (Say-on-Frequency)6 (Say-on-Pay). Broker non-votes will have no effect on the outcomes of Proposal 1 Proposal 2, Proposal 3, Proposal 4 or Proposal 5.6.

All outstanding shares of our common stock represented by valid and unrevoked proxies received in time for the 20202021 Annual Meeting will be voted. With respect to the election of directors, a stockholder may (i) vote for the election of the director nominee,nominees, or (ii) withhold authority to vote for suchone or both director nominee.nominees.

The proposals (x)(A) to approve an amendment to our Restated Certificate of Incorporation to effect a reverse stock splitincrease the number of ourauthorized shares of common stock at a ratio infrom 200,000,000 to 400,000,000, (B) to approve an amendment to our 2014 Incentive Plan to increase the rangenumber of 1:3shares of common stock reserved for issuance under the 2014 Incentive Plan by 12,000,000 shares of common stock, (C) to 1:10, with such ratioapprove an amendment to be determined inour 2014 ESPP to increase the discretionnumber of our Board, (y)shares of common stock reserved for issuance under the 2014 ESPP by 2,300,000 shares of common stock, (D) to ratify Ernst & Young LLP as our independent registered public accounting firm for the fiscal year ending December 31, 2020,2021, and (z)(E) to approve, on a non-binding advisory basis, the compensation of our named executive officers, 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 theour Board’s recommendations as follows: (i) “FOR” the election of the named director nominee;nominees; (ii) “FOR” approval of the

This Proxy Statementproxy statement and accompanying form of proxy card are first being mailed or given to stockholders on or about , 2020.

Vice President, Chief Financial Officer and Head of Business Development of Axerion Pharmaceuticals, Inc., a pharmaceutical company, from September 2009 to June 2010, provided independent consulting services from February 2009 to September 2009 and served as the Executive Vice President, Chief Financial Officer and Chief

In addition, the Nasdaq Stock Market Rules require that, subject to specified exceptions, (i) each member of our audit and compensation committees be independent; (ii) each member of our audit committee satisfy independence criteria set forth in Rule 10A-3 under the Securities Exchange Act of 1934, as amended, or the Exchange Act; and (iii) each member of our compensation committee satisfy the independence criteria set forth in Rule 10C-1 under the Exchange Act. In order to be considered independent for purposes of Rule 10A-3, a member of our audit committee may not, other than in his or her capacity as a member of the audit committee, the Board, or any other Board committee: (1) accept, directly or indirectly, any consulting, advisory or other compensatory fee from us or any of our subsidiaries; or (2) be an affiliated person of us or any of our subsidiaries. In addition, Rule 10C-1 under the Exchange Act requires that, with respect to the independence of each member of our compensation committee, our Board consider all factors specifically relevant to determining whether a director has a relationship with us which is material to that director’s ability to be independent from management in connection with his or her duties as a compensation committee member, including, but not limited to: (1) the source of his or her compensation as a director, including any consulting, advisory or other compensatory fee paid by us to him or her; and (2) whether he or she is affiliated with us or any of our subsidiaries or affiliates.

Our Board has also determined that: (i) Ms. Henderson, Mr. LynchKeyes and Mr. Keyes,Ms. Bourdow, who comprise our audit committee; and (ii) Mr. Duker, Ms. Henderson and Mr. Lynch,Keyes, who comprise our compensation committee, each satisfy the independence standards for those committees established by the applicable rules and regulations under the Exchange Act and the Nasdaq Stock Market Rules. In addition, each member of our nominating and corporate governance committee is independent as defined under the Nasdaq Stock Market Rules. In making such determination, our Board considered the relationships that each such non-employee director has with us and all other facts and circumstances our Board deemed relevant in determining independence.

No director is related to any of our other directors or executive officers and there are no arrangements or understandings between a director and any other person pursuant to which such person was elected as director.

Our Board decided to separate the roles of chair and chief executive officer because it believes that a bifurcated leadership structure offers the following benefits:

Our Board has determined that each of Ms. Henderson, Mr. LynchKeyes and Mr. KeyesMs. Bourdow is an “audit committee financial expert” as defined in applicable SEC rules and that each qualifies as independent as defined under the applicable Nasdaq rules.Stock Market Rules.

The audit committee met five times during 2019.2020.

The processes and procedures followed by our compensation committee in considering and determining executive and director compensation are described below under “Board Processes-Executive and Director Compensation Processes.” In addition to the Board’s independence determination, our Board has determined that each member of our compensation committee is a “non-employee director” for purposes of Rule 16b-3 under the Exchange Act.

The compensation committee met sixfour times during 2019.2020.

The nominating and corporate governance committee met one time during 2019.did not hold any formal meetings in 2020 and acted by unanimous written consent twice in 2020.

At the 20202021 Annual Meeting, stockholders will be asked to consider the election of Dr. Cannell and Ms. Henderson.Bourdow.

Our Board met eleven times during 2019.2020. During 2019,2020, each director attended at least 75% of the aggregate of total the number of Board meetings and the number of meetings held by all committees of the Board on which he or she then served, except for Mr. Dan who attended four of the six (66.7%) Board meetings held in 2019 before his retirement from the Board on July 16, 2019.served.

Our directors are encouraged to attend our annual meetings of stockholders. At the annual meeting of stockholders held in 2019, five2020, three of the sixfive directors then in office were in attendance.

In considering whether to recommend any particular candidate for inclusion in our Board’s slate of recommended director nominees, our nominating and corporate governance committee and our Board apply the criteria set forth in our corporate governance guidelines described below under “Corporate Governance Guidelines.” Consistent with these criteria, our nominating and corporate governance committee and our Board expect every nominee to have the following attributes or characteristics, among others: integrity, honesty, adherence to high ethical standards, business acumen, good judgment and a commitment to understand our business and industry.

The director nominee,nominees, Dr. Cannell and Ms. Henderson, isBourdow, are currently a membermembers of our Board. The nominee's biographynominee biographies under “Board of Directors - Members of Our Board of Directors”Directors�� indicate the experience, qualifications, attributes and skills of the nominee, Ms. Henderson,each of our director nominees that led our nominating and corporate governance committee and our Board to conclude shethat each of Dr. Cannell and Ms. Bourdow should continue to serve as a member of our Board. Our nominating and corporate governance committee and our Board believe that each of the nominee, Ms. Henderson,nominees has the individual attributes and characteristics required of each of our directors, and that the nominee possessesnominees as a group possess the skill sets and specific experience desired of our Board as a whole.

Our nominating and corporate governance committee and our Board consider the value of diversity when selecting nominees, and believe that our Board, taken as a whole, should embody a diverse set of skills, experiences and abilities. Our nominating and corporate governance committee and our Board do not make any particular weighting of diversity or any other characteristic in evaluating nominees and directors.

Stockholders may recommend individuals for consideration as potential director candidates by submitting their names, together with appropriate biographical information and background materials, and information with respect to the stockholder or group of stockholders making the recommendation, including the number of shares of common stock owned by such stockholder or group of stockholders, to our Corporate Secretary at Sesen Bio, Inc., 245 First Street, Suite 1800, Cambridge, MA 02142, Attention: Corporate Secretary. The specific requirements for the information that is required to be provided for such recommendations to be considered are specified in our by-lawsAmended and Restated By-laws and must be received by us no later than the date referenced below in “Other Matters—Deadline for Submission of Stockholder Proposals for the 20212022 Annual Meeting of Stockholders.Meeting.” Assuming that appropriate biographical and background material has been provided on a timely basis, the nominating and corporate governance committee and our Board will evaluate stockholder-recommended candidates by following substantially the same process, and applying substantially the same criteria, as it follows for candidates submitted by others.

Stockholders also have the right under our by-lawsAmended and Restated By-laws to directly nominate director candidates, without any action or recommendation on the part of the nominating and corporate governance committee or our Board, by following the procedures set forth under “Other Matters—Deadline for Submission of Stockholder Proposals for the 20212022 Annual Meeting of Stockholders.Meeting.”

Our executive compensation program is administered by the compensation committee of our Board, subject to the oversight 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.

In designing our executive compensation program, our compensation committee considers publicly available compensation data for national and regional companies in the biotechnology/pharmaceutical industry to help guide its executive compensation decisions at the time of hiring and for subsequent adjustments in compensation. Our compensation committee has also retained the services of Radford, an independent compensation consultant, to provide it with additional comparative data on executive compensation practices in our industry and to advise us on our executive compensation program generally. Although the compensation committee considers Radford’s advice and recommendations about our executive compensation program, the compensation committee ultimately makes its own decisions about these matters.

None of the compensation committee members and none of our executive officers or directors have any relationship with Radford or the individual consultants employed by Radford. Radford has not provided any other services to our Board or management other than compensation consulting services to the compensation committee. The compensation committee has determined that no conflicts of interest exist between Radford and our company, our directors or our executive officers. The compensation committee is directly responsible for the appointment and oversight of any compensation consultants and other advisors it retains.

Our director compensation program is administered by the compensation committee of our Board, subject to the oversight and approval of our full Board. The compensation committee conducts periodic reviews of director compensation and makes recommendations to the Board with respect thereto.

A copy of the corporate governance guidelines is posted under the heading “Corporate Governance” on the Investor Relations section of our website, which is located at www.sesenbio.com.

Our Board has adopted written policies and procedures for the review of any transaction, arrangement or relationship in which our company is a participant, 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 and the amount involved in such transaction, arrangement or relationship exceeds the lesser of $120,000 and one percent of the average of our total assets at year-end for the last two completed fiscal years. Because one percent of our average total assets for the past two fiscal years has exceeded $120,000, our Board applies the $120,000 threshold under our policy.

If a related person proposes to enter into such a transaction, arrangement or relationship, which we refer to as a “related person transaction,” the related person must report the proposed related person transaction to our principal financial officer. The policy calls for the proposed related person transaction to be reviewed and, if deemed appropriate, approved by our audit committee. Whenever practicable, the reporting, review and approval will occur prior to entry into the transaction. If advance review and approval is not practicable, the committee will review, and, in its discretion, may ratify the related person transaction or direct that such transaction be terminated and rescinded. The policy also permits the chair of the audit committee to review and, if deemed appropriate, approve proposed related person transactions that arise between committee meetings, subject to ratification by the audit committee at its next meeting. Any related person transactions that are ongoing in nature will be reviewed annually.

A related person transaction reviewed under the policy will be considered approved or ratified if it is authorized by the audit committee after full disclosure of the related person’s interest in the transaction. As appropriate for the circumstances, the audit committee will review and consider:

Our audit committee may approve or ratify the transaction only if it determines that, under all of the circumstances, the transaction is in, or is not inconsistent with, our best interests. Our audit committee may impose any conditions on the related person transaction that it deems appropriate.

In addition to the transactions that are excluded by the instructions to the SEC’s related person transaction disclosure rule, our Board has determined that the following transactions do not create a material direct or indirect interest on behalf of related persons and, therefore, are not related person transactions for purposes of this policy:

The policy provides that transactions involving the compensation of executive officers shall be reviewed and approved by our compensation committee in the manner specified in the compensation committee’s charter.

We are party to an intellectual property license agreement under which we pay fees to Protoden Technologies Inc., or Protoden, a company owned by an affiliate of Mr. Dan. Pursuant to the agreement, we have an exclusive, perpetual, irrevocable and non-royalty bearing license, with the right to sublicense, under certain patents and technology to make, use and sell products that utilize such patents and technology. The annual fee is $100,000. Beginning on January 1, 2025, the licenses granted to us will require no further payments to Protoden. We paid $100,000 under this license agreement to Protoden in each of the years ended December 31, 20182020 and 2019.

In addition to the biographical information for Dr. Cannell, which is set forth above, under “Corporate Governance - Board of Directors - Members of Our Board of Directors,” set forth below is certain biographical information about Ms. Forbes, Dr. MacDonald and Mr. Sullivan:

Our named executive officers for 20192020 are Thomas R. Cannell, D.V.M., our President and Chief Executive Officer, Monica Forbes, our Chief Financial Officer, and Mark R. Sullivan,Glen MacDonald, our General Counsel and Corporate Secretary.Chief Technology Officer.