AMENDED & RESTATED EXECUTIVE EMPLOYMENT AGREEMENT

Exhibit 10.48

AMENDED & RESTATED EXECUTIVE EMPLOYMENT AGREEMENT

THIS AMENDED & RESTATED EXECUTIVE EMPLOYMENT AGREEMENT (“ Agreement ”) is made and entered into as of the 15th day of December, 2008, by and between SEATTLE GENETICS, INC., a Delaware corporation (“ Company ”) and Clay B. Siegall (“ Executive ”).

RECITALS:

WHEREAS , Executive and Company are parties to the Executive Employment Agreement dated October 26, 2001, as amended (the “ Prior Agreement ”) and wish to amend and restate the Prior Agreement to clarify certain existing provisions in light of final regulations issued under Section 409A of the Internal Revenue Code of 1986, as amended (the “ Code ”).

NOW, THEREFORE, in consideration of the mutual covenants and agreements contained in this Agreement, the sufficiency of which is hereby acknowledged, the parties agree as follows:

1.1 Title and Responsibilities. Executive shall serve as President and Chief Executive Officer of the Company. Executive’s responsibilities and duties shall include those inherent in Executive’s position with the Company and shall further include such other managerial responsibilities and executive duties consistent with such position as may be assigned to Executive from time to time by the Chairman of the Board or the Board of Directors of the Company. Executive shall devote his best efforts and full business time to the business and interests of the Company. During the term of Executive’s employment with the Company, Executive may serve on the board of directors of other companies, manage personal investments, and engage in civic and charitable activities, provided that such activities shall not represent a conflict of interest with the Company and do not materially detract from fulfilling Executive’s responsibilities and duties to the Company.

1.2 Board of Directors. The Board of Directors of the Company shall take whatever steps are necessary to continue to nominate Executive for election to the Board of Directors of the Company in every election of Executive’s class of directors presented to stockholders following execution of this Agreement.

2.1 Base Salary. Executive shall be paid a base salary (“ Base Salary ”) by the Company during the term of Executive’s employment at the rate determined by the Compensation Committee of the Board of Directors (the “ Compensation Committee ”), which is currently $575,000.00 per year. Executive’s Base Salary shall be reviewed annually by the Compensation Committee and evaluated based on performance and salary levels of other executives of comparable position within the industry and geographic location of the Company. Based upon

such evaluation and review, Executive’s Base Salary may be adjusted from time to time as determined by the Compensation Committee in its sole discretion.

2.2 Bonus. Executive may be eligible to receive an annual bonus (“ Annual Bonus ”), currently targeted at fifty percent (50%) of Executive’s Base Salary, based upon performance criteria and financial and operational results of the Company as determined by the Compensation Committee. To the extent that the Annual Bonus is earned and becomes payable in accordance with the terms under which it is offered and unless otherwise specified in a written document reflecting the bonus arrangement, any Annual Bonus earned by Executive will be paid to Executive prior to two and one half (2  ¹ / 2 ) months following the year in which the Annual Bonus becomes payable as a result of Executive’s vesting in the right to the Annual Bonus.

2. 3 Stock Options . Executive may be eligible to receive grants of stock options or purchase rights from time to time in the future, on such terms and subject to such conditions as the Compensation Committee shall determine as of the date of any such grant and pursuant to the existing stock plan(s) of the Company.

2.4 Other Benefits.

(i) Executive shall be entitled to such employee benefits generally available to full-time salaried employees of the Company, including without limitation, health insurance, paid vacation of not less than four (4) weeks per year, retirement plans and other similar benefits; provided, that Company reserves the right to amend, modify, terminate or make any other changes in such benefits generally available to full-time salaried employees of the Company at any time in its sole discretion.

(ii) The Company shall pay or reimburse Executive for all travel and entertainment expenses incurred by Executive in connection with Executive’s duties on behalf of the Company, subject to the reasonable approval of the Company. Executive shall only be entitled to reimbursement to the extent that Executive follows the reasonable procedures established by the Company for reimbursement of such expenses which will include, but will not be limited to, providing satisfactory evidence of such expenditures.

3.1 Termination of Employment and Severance Benefits .

(a) Termination of Employment . This Agreement may be terminated upon the occurrence of any of the following events:

(i) The Company’s determination in good faith that it is terminating Executive for Cause (as defined in Section 3.3 below) (“ Termination for Cause ”);

(ii) The Company’s determination that it is terminating Executive without Cause, which determination may be made by the Company at any time at the Company’s sole discretion, for any or no reason (“ Termination Without Cause ”);

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(iii) The effective date of a written notice sent to the Company from Executive stating that Executive is electing to terminate his employment with the Company (“ Voluntary Termination ”);

(iv) A change in Executive’s status such that a Constructive Termination (as defined in Section 3.2(d) below) has occurred; or

(v) Following Executive’s death or Disability (as defined in Section 3.4 below).

3.2 Severance Benefits . Executive shall be entitled to receive severance benefits upon termination of employment only as set forth in this Section 3.2 contingent upon resignation from all positions held by Executive and only if Executive executes a full release and waiver of claims within thirty (30) days of Executive’s termination (and allows it to become effective in accordance with its terms):

(a) Voluntary Termination. If Executive’s employment terminates by Voluntary Termination, then Executive shall not be entitled to receive payment of any severance benefits. Executive will receive payment(s) for all salary and unpaid vacation accrued as of the date of Executive’s termination of employment and Executive’s benefits will be continued under the Company’s then existing benefit plans and policies in accordance with such plans and policies in effect on the date of termination and in accordance with applicable law.

(b) Involuntary Termination. If Executive’s employment is terminated under Section 3.1(a)(ii) (Termination Without Cause) or 3.1(a)(iv) (Constructive Termination) above (such termination, an “ Involuntary Termination ”), Executive will be entitled to receive payment of severance benefits equal to Executive’s regular monthly salary for twelve (12) months (the “ Severance Period ”); provided, that if the Involuntary Termination occurs within twelve (12) months after a Change in Control (as defined below), such Severance Period shall be for a period of twenty-four (24) months. Such payments shall be made, at the Company’s option, in a lump sum within thirty (30) days after the date of Executive’s Involuntary Termination or periodically over the Severance Period according to the Company’s standard payroll schedule, provided that such payments may not extend beyond two and one-half (2  ¹ / 2 ) months following the end of the calendar year in which the date of Involuntary Termination occurs. Executive will also be entitled to receive payment on the date of Involuntary Termination of the pro rata portion of any Annual Bonus based on achievement of the specific corporate and individual performance targets established for the fiscal year in which the termination occurs, payable prior to two and one-half (2  ¹ / 2 ) months following the end of the calendar year in which the date of Involuntary Termination occurs. Executive will receive payment(s) for all salary and unpaid vacation accrued as of the date of Executive’s termination of employment and health insurance benefits will be continued through payment of Executive’s COBRA health insurance premiums by the Company over the Severance Period so long as Executive timely elects to continue Executive’s health insurance coverage under COBRA and subject to COBRA’s terms, conditions and requirements.

(c) Termination for Cause. If Executive’s employment is terminated for Cause, then Executive shall not be entitled to receive payment of any severance benefits.

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Executive will receive payment(s) for all salary and unpaid vacation accrued as of the date of Executive’s termination of employment and Executive’s benefits will be continued under the Company’s then existing benefit plans and policies in accordance with such plans and policies in effect on the date of termination and in accordance with applicable law.

(d) Constructive Termination. “ Constructive Termination ” shall be deemed to occur if (A) there is a material reduction or change in job duties, responsibilities and requirements inconsistent with Executive’s position with the Company and prior duties, responsibilities and requirements, provided that neither a mere change in title alone nor reassignment to a position that is substantially similar to the position held prior to the change in terms of job duties, responsibilities or requirements shall constitute a material reduction in job responsibilities; or (B) there is a reduction in Executive’s then-current base salary by at least twenty percent (20%), provided that an across-the-board reduction in the salary level of all other senior executives by the same percentage amount as part of a general salary level reduction shall not constitute such a salary reduction; or (C) Executive refuses to relocate to a facility or location more than 50 miles from the Company’s current location; provided, however, that in each case above, Executive must first provide notice of the existence of the circumstances giving rise to a Constructive Termination within ninety (90) days of the initial existence of such circumstances and the Company must be provided with a period of thirty (30) days from the date of receipt of such notice to cure the circumstances giving rise to a Constructive Termination; provided further that the Company may notify Executive at any time prior to expiration of the cure period that it will not cure the circumstances, in which case the cure period shall end immediately upon such notification.

(e) Termination by Reason of Death or Disability. In the event that Executive’s employment with the Company terminates as a result of Executive’s death or Disability (as defined in Section 3.4 below), Executive or Executive’s estate or representative will receive all salary and unpaid vacation accrued as of the date of Executive’s death or Disability and any other benefits payable under the Company’s then existing benefit plans and policies in accordance with such plans and policies in effect