Exhibit 99.1

Seattle Genetics Reports Second Quarter 2015 Financial Results

-Total Second Quarter 2015 Revenues of $77.1 Million, Including Record $55.1 Million in

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

-ADCETRIS 2015 Net Sales Guidance Increased to a Range of $210 Million to $220 Million-

-Potential ADCETRIS Label Expansion through AETHERA Supplemental Biologics License Application (BLA) Currently Under FDA Priority Review; PDUFA Action Date August 18, 2015-

-Additional Clinical Data from Multiple Pipeline Programs Expected In Next Six to Nine Months, Including SGN-CD33A for Acute Myeloid Leukemia-

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

Bothell, WA — July 30, 2015 — Seattle Genetics, Inc. (Nasdaq: SGEN) today reported financial results for the second quarter and six months ended June 30, 2015. The company also highlighted ADCETRIS (brentuximab vedotin) commercialization, regulatory and clinical development accomplishments, progress with other proprietary pipeline programs and antibody-drug conjugate (ADC) and other collaborator updates.

“With record ADCETRIS net sales in the second quarter and a broad ongoing clinical development program comprising four phase 3 trials for a range of CD30-expressing malignancies, we are enthusiastic about the potential to expand ADCETRIS to help patients in need,” said Clay Siegall, Ph.D., President and Chief Executive Officer of Seattle Genetics. “Looking ahead, we are increasing our 2015 ADCETRIS net sales guidance by $10 million to now be in the range of $210 million to $220 million. We anticipate a decision from the FDA on our AETHERA supplemental BLA by August 18^th, and are on track to complete enrollment in our phase 3 ECHELON-1 and ALCANZA trials before the end of 2015. In addition, our clinical-stage pipeline is gaining increased visibility with data from multiple programs expected over the course of the next six to nine months, notably SGN-CD33A and SGN-CD19A.”

Recent ADCETRIS Highlights

Recent Pipeline, Collaborator and Other Highlights

Anticipated Upcoming Activities

ADCETRIS is not currently approved for the post-transplant consolidation treatment of HL patients at high risk of relapse or progression, or for use in DLBCL, SLE or non-Hodgkin lymphoma other than systemic anaplastic large cell lymphoma.

Second Quarter and Six Months 2015 Financial Results

Total revenues in the second quarter and six month periods ended June 30, 2015 increased to $77.1 million and $159.3 million, respectively, from $68.3 million and $136.6 million for the same periods in 2014. ADCETRIS sales in the second quarter were $55.1 million, compared to $44.8 million for the second quarter of 2014. For the year-to-date, ADCETRIS sales were $104.0 million, compared to $83.5 for the year-to-date period in 2014, an increase of 25 percent. Second quarter 2015 revenues also included royalty revenues driven by international sales of ADCETRIS by Takeda of $7.6 million, compared to $7.3 million in the second quarter of 2014. For the year-to-date in 2015, royalty revenues were $18.7 million, compared to $20.0 million for the first six months of 2014. First quarter 2014 royalty revenues included a $5 million sales milestone payment from Takeda. In addition, revenues included amounts earned under the company’s ADCETRIS and ADC collaborations totaling $14.4 million in the second quarter and $36.6 million for the first six months of 2015, compared to $16.2 million and $33.1 million for the same periods in 2014.

Total costs and expenses for the second quarter of 2014 were $124.7 million, compared to $86.0 million for the second quarter of 2014. For the first six months of 2015, total costs and expenses were $228.6 million, compared to $170.6 million in the first six months of 2014. The increase in 2015 costs and expenses was primarily driven by the $25.0 million upfront payment to Unum and investment in Seattle Genetics’ pipeline programs.

Non-cash, share-based compensation cost for the first six months of 2015 was $17.6 million, compared to $18.7 million for the first six months of 2014.

Net loss for the second quarter of 2015 was $47.5 million, or $0.38 per share, compared to a net loss of $17.6 million, or $0.14 per share, for the second quarter of 2014. For the six months ended June 30, 2015, net loss was $69.2 million, or $0.55 per share, compared to a net loss of $33.9 million, or $0.28 per share, for the same period in 2014.

As of June 30, 2015, Seattle Genetics had $249.5 million in cash, cash equivalents and investments, compared to $313.4 million as of December 31, 2014.

2015 Financial Outlook

Seattle Genetics anticipates that 2015 revenues from ADCETRIS net product sales in the U.S. and Canada will be slightly higher than previously anticipated, and are now expected to be in the range of $210 million to $220 million. The company also now anticipates that 2015 research and development expenses will be in the range of $275 million to $300 million, primarily due to the $25 million upfront payment under the recent collaboration with Unum Therapeutics.

Conference Call Details

Seattle Genetics’ management will host a conference call and webcast to discuss the 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 Seattle Genetics’ website at www.seattlegenetics.com, under the Investors and News section, or by calling 888-632-3384 (domestic) or 785-424-1675 (international). The conference ID is 7873635. A replay of the discussion will be available beginning at approximately 4:30 p.m. PT today from Seattle Genetics’ website or by calling 888-203-1112 (domestic) or 719-457-0820 (international), using conference ID 7873635. The telephone replay will be available until 5:00 p.m. PT on Monday, August 3, 2015.

About Seattle Genetics

Seattle Genetics is a biotechnology company focused on the development and commercialization of innovative antibody-based therapies for the treatment of cancer. Seattle Genetics is leading the field in developing antibody-drug conjugates (ADCs), a technology designed to harness the targeting ability of antibodies to deliver cell-killing agents directly to target cells. The company’s lead product, ADCETRIS^® (brentuximab vedotin) is a CD30-targeted ADC that, in collaboration with Takeda Pharmaceutical Company Limited, is commercially available for two indications in more than 55 countries, including the U.S., Canada, Japan and members of the European Union. Additionally, ADCETRIS is being evaluated broadly in more than 30 ongoing clinical trials in

CD30-expressing malignancies. Seattle Genetics is also advancing a robust pipeline of clinical-stage programs, including SGN-CD19A, SGN-CD33A, SGN-LIV1A, SGN-CD70A, ASG-22ME, ASG-15ME and SEA-CD40. Seattle Genetics has collaborations for its ADC technology with a number of leading biotechnology and pharmaceutical companies, including AbbVie, Agensys (an affiliate of Astellas), Bayer, Genentech, GlaxoSmithKline and Pfizer. More information can be found at www.seattlegenetics.com.

Certain of the statements made in this press release are forward looking, such as those, among others, relating to the company’s expectations for 2015 ADCETRIS net sales and 2015 research and development expenses, upcoming clinical activities, including enrollment completion and data availability from ongoing clinical trials and the timing thereof, and the initiation of future clinical trials, the potential approval by the FDA of the company’s supplemental BLA for ADCETRIS in the AETHERA treatment setting and the timing of the approval decision by the FDA, the opportunities for, and the therapeutic and commercial potential of, ADCETRIS and the company’s 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 risk that sales of ADCETRIS and our research and development expenses may not meet or exceed our guidance or otherwise be as we expect, including, with respect to the company’s 2015 ADCETRIS net sales guidance, that ADCETRIS may not be approved by the FDA in the AETHERA treatment setting in a timely manner or at all, which could negatively impact anticipated ADCETRIS net sales, as well as other risks related to future opportunities and plans, including the uncertainty of expected future financial performance and results. We may also be delayed in our planned clinical trial initiations, the enrollment in and conduct of our clinical trials, and obtaining data from clinical trials, in each case for a variety of reasons, including the difficulty and uncertainty of pharmaceutical product development. We may also be unable to expand ADCETRIS’ labeled indications, including for use in the AETHERA treatment setting, or complete the development of, and obtain regulatory approval for, our product candidates, each of which are in relatively early stages of development. More information about the risks and uncertainties faced by Seattle Genetics is contained in the company’s Quarterly Report on Form 10-Q for the quarter ended March 31, 2015 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:

Tricia Larson

(425) 527-4180

tlarson@seagen.com

Seattle Genetics, Inc.

Condensed Consolidated Balance Sheets

(Unaudited)

(In thousands)

5

Seattle Genetics, Inc.

Condensed Consolidated Statements of Operations

(Unaudited)

(In thousands, except per share amounts)

6