Exhibit 99.1

Seattle Genetics Reports Third Quarter 2017 Financial Results

-Third Quarter 2017 Revenues Were $135.3 Million, Including $79.2 Million in

ADCETRIS^® (Brentuximab Vedotin) U.S. and Canada Net Sales-

-FDA Breakthrough Therapy Designation Granted for ADCETRIS in Combination with

Chemotherapy for Frontline Advanced Classical Hodgkin Lymphoma; Supplemental Biologics

License Application on Track for Submission in the Fourth Quarter of 2017-

-Full Data from ADCETRIS Phase 3 ECHELON-1 Trial Accepted for Presentation at ASH

Annual Meeting-

-Pivotal Trial Initiated of Enfortumab Vedotin in Locally Advanced or Metastatic Urothelial

Cancer-

-Conference Call Today at 4:30 p.m. ET-

Bothell, WA — October 26, 2017 — Seattle Genetics, Inc. (NASDAQ: SGEN) today reported financial results for the third quarter and nine months ended September 30, 2017. The company also highlighted ADCETRIS (brentuximab vedotin) commercialization and clinical development accomplishments, enfortumab vedotin (ASG-22ME) and tisotumab vedotin clinical activities, as well as progress with its pipeline of antibody-drug conjugates (ADCs) and other proprietary programs.

“We have recently delivered on several important goals that continue to strengthen the ADCETRIS brand and advance our portfolio of pipeline programs. The FDA granted Breakthrough Therapy Designation to ADCETRIS in combination with chemotherapy in frontline advanced classical Hodgkin lymphoma based on data from the phase 3 ECHELON-1 clinical trial, and we are on track to submit a supplemental Biologics License Application (BLA) for this indication soon. And, the supplemental BLA for ADCETRIS in cutaneous T-cell lymphoma was filed by the FDA with priority review and a PDUFA date of December 16, 2017,” said Clay Siegall, Ph.D., President and Chief Executive Officer of Seattle Genetics. “We have also made significant progress with our pipeline, which now includes two ADCs for which we are pursuing rapid development pathways based on positive FDA feedback. We and our partner Astellas have initiated a pivotal phase 2 trial of enfortumab vedotin in metastatic urothelial cancer, and we and our partner Genmab plan to advance tisotumab vedotin into a pivotal phase 2 trial for recurrent or metastatic cervical cancer in the first half of 2018. We anticipate several additional milestones before the end of 2017, including a strong presence at the American Society of Hematology annual meeting in December highlighted by a presentation of our full ECHELON-1 data in frontline Hodgkin lymphoma.”

ADCETRIS Program Highlights

ADCETRIS is not currently approved for use in CTCL, frontline Hodgkin lymphoma or frontline MTCL.

Enfortumab Vedotin Program Highlights

Tisotumab Vedotin Program Highlights

Other Recent Activities

Third Quarter and Nine Months 2017 Financial Results

Total revenues in the third quarter and nine month periods ended September 30, 2017 increased to $135.3 million and $352.6 million, respectively, compared to $106.3 million and $312.9 million from the same periods in 2016. Revenues included:

Total costs and expenses for the third quarter of 2017 were $167.5 million, compared to $138.7 million for the third quarter of 2016. For the first nine months of 2017, total costs and expenses were $503.4 million, compared to $399.7 million in the first nine months of 2016. The increase in 2017 costs and expenses was primarily driven by ADCETRIS drug supply provided to Takeda as well as investment in enfortumab vedotin, SGN-LIV1A and the company’s pipeline programs.

Non-cash, share-based compensation cost for the first nine months of 2017 was $47.9 million, compared to $37.0 million for the same period in 2016.

As part of a previously terminated license transaction with Immunomedics, Seattle Genetics holds 3.0 million shares of Immunomedics common stock and a warrant to purchase an additional 8.7 million shares. In September 2017, Immunomedics’ resale registration statement covering the shares issuable upon exercise of the warrant was declared effective by the SEC. As a result, the warrant is accounted for as a derivative security and marked-to-market. This led to a gain of $78.7 million in investment and other income, net, recorded in the quarter ended September 30, 2017.

Due to the gain on the Immunomedics warrant, Seattle Genetics reported net income in the third quarter of 2017 of $50.0 million, or $0.34 per fully diluted share, compared to a net loss of $31.8 million, or $0.23 per share, for the third quarter of 2016. For the nine months ended September 30, 2017, net loss was $66.3 million, or $0.46 per share, compared to a net loss of $85.0 million, or $0.61 per share, for the nine months period ended September 30, 2016.

As of September 30, 2017, Seattle Genetics had $470.4 million in cash, cash equivalents and investments.

2017 Financial Outlook

Seattle Genetics updated its full year 2017 financial guidance as follows:

Conference Call Details

Seattle Genetics’ management will host a conference call and webcast to discuss its third quarter 2017 financial results and provide an update on business activities. The event will be held today at 1:30 p.m. Pacific Time (PT); 4:30 p.m. Eastern Time (ET). The live event will be available from the Seattle Genetics website at www.seattlegenetics.com, under the Investors section, or by calling 877-830-2649 (domestic)

or 785-424-1824 (international). The conference ID is 4819463. A replay of the discussion will be available beginning at approximately 4:30 p.m. PT today from the Seattle Genetics website or by calling 888-203-1112 (domestic) or 719-457-0820 (international), using conference ID 4819463. The telephone replay will be available until 5:00 p.m. PT on Monday, October 30, 2017.

About Seattle Genetics

Seattle Genetics is an innovative biotechnology company dedicated to improving the lives of people with cancer through novel antibody-based therapies. The company’s industry-leading antibody-drug conjugate (ADC) technology harnesses the targeting ability of antibodies to deliver cell-killing agents directly to cancer cells. Seattle Genetics commercializes ADCETRIS^® (brentuximab vedotin) for the treatment of several types of CD30-expressing lymphomas. The company is also advancing a robust pipeline of novel therapies for solid tumors and blood-related cancers designed to address significant unmet medical needs and improve treatment outcomes for patients. More information can be found at www.seattlegenetics.com and follow @SeattleGenetics on Twitter.

Forward-Looking Statements

Certain of the statements made in this press release are forward looking, such as those, among others, relating to the company’s 2017 outlook, including anticipated 2017 revenues, costs and expenses; the company’s potential to achieve a series of regulatory and commercial milestones in 2017 and 2018; anticipated regulatory events and clinical activities, including data availability from ECHELON-2, as well as other planned and ongoing clinical trials, including clinical trials of enfortumab vedotin and tisotumab vedotin, the timing of these clinical trials, and their possible utility to support regulatory approval of the respective product candidates; the anticipated submission of a supplemental BLA for label expansion of ADCETRIS in frontline Hodgkin lymphoma and the timing thereof and other anticipated regulatory milestones, including any action by the FDA with respect to the supplemental BLA submitted for label expansion of ADCETRIS based on the ALCANZA trial; the opportunities for, and the therapeutic and commercial potential of, ADCETRIS, enfortumab vedotin, tisotumab vedotin, SGN-LIV1A and the company’s other product candidates; as well as other statements that are not historical facts. Actual results or developments may differ materially from those projected or implied in these forward-looking statements. Factors that may cause such a difference include the risks that the company’s 2017 ADCETRIS net sales and financial guidance may not be as expected. The company may also be delayed in its planned clinical trial initiations, the enrollment in and conduct of its clinical trials, obtaining data from clinical trials, planned regulatory submissions, and regulatory approvals in each case for a variety of reasons including the difficulty and uncertainty of pharmaceutical product development, unexpected adverse events or regulatory discussions or actions, including with respect to special protocol assessment agreements defining trial endpoints and the inherent uncertainty associated with the regulatory approval process. The company may also be unable to expand ADCETRIS’ labeled indications due to unexpected or delayed data from its ongoing phase 3 trials or regulatory action, including the risks that the company’s supplemental BLA submissions may not be accepted for filing by, or ultimately approved by, the FDA in a timely manner or at all or with the requested label, and may be unable to complete the development of, and obtain regulatory approval for, its product candidates. More information about the risks and uncertainties faced by Seattle Genetics is contained under the caption “Risk Factors” included in the company’s Quarterly Report on Form 10-Q for the quarter ended June 30, 2017 filed with the Securities and Exchange Commission. Seattle Genetics disclaims any intention or obligation to update or revise any forward-looking statements, whether as a result of new information, future events or otherwise.

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CONTACTS:

Investors:

Peggy Pinkston

(425) 527-4160

ppinkston@seagen.com

Media:

Brandi Robinson

(425) 527-2910

brobinson@seagen.com

Seattle Genetics, Inc.

Condensed Consolidated Balance Sheets

(Unaudited)

(In thousands)

Seattle Genetics, Inc.

Condensed Consolidated Statements of Operations

(Unaudited)

(In thousands, except per share amounts)