Associate Attorney General Tony West, the Justice Department’s third-ranking official and a key player in the agency’s effort to secure multibillion-dollar penalties against large banks for financial fraud, is stepping down, officials said.

West, 49, declined to say who might replace him or where he is going, during an interview in his fifth-floor office, which is decorated with a large portrait of former attorney general Robert F. Kennedy and a photograph of the African American Buffalo Soldiers Army regiment. His resignation is effective Sept. 15.

West said his most meaningful accomplishment during 51 / 2 years in the Obama administration was his role in the Justice Department’s decision to stop backing the Defense of Marriage Act.

“It was one of the hardest things that I’ve ever been involved in and it is one of the things that I think will matter the most,” West said.

He recalled phoning lawyer Roberta A. Kaplan, who represented Edith Windsor in what would be the Supreme Court’s historic ruling ordering the federal recognition of same-sex marriage, to tell her of the agency’s decision to no longer defend the law. “She started to cry on the phone,” he said.

Tony West said his most meaningful accomplishment as associate attorney general in the Justice Department involved defeating the Defense of Marriage Act. (Marvin Joseph/The Washington Post)

“Other than the attorney general, the most important person who helped shape the outcome of that case was Tony West,” said Labor Secretary Thomas Perez, who previously headed the civil rights division at the Justice Department.

Federal prosecutors nationwide said West’s strength was his ability to bring people together and bridge the “disconnect” that often exists between Washington officials and the 93 U.S. attorneys in the field.

“Tony’s attitude has never been, ‘We don’t do it that way,’ ” said Loretta E. Lynch, the U.S. attorney for the Eastern District of New York. “It has always been, ‘Why don’t we try it?’ ”

West said he also is proud of his role in leading the department’s efforts to combat financial fraud, securing record-breaking civil penalties and global settlements, including a $16.65 billion settlement with Bank of America and a $13 billion settlement with JPMorgan Chase.

Questions remain about whether enough of the money from the settlements will make its way to investors who bought faulty mortgage securities. But West said the department recovered nearly $37 billion for American consumers and investors harmed by the financial crisis.

“I think there are a lot of people who will be able to stay in their homes and have a second chance at having the American dream because of some of the work we were able to do,” he said.

West also was central to the Obama administration’s efforts in Indian country, overseeing the creation and work of the Task Force on American Indian and Alaska Native Children Exposed to Violence.

He cited the Justice Department’s work for the reauthorization of the Violence Against Women Act, which for the first time will allow Indian tribes to prosecute certain crimes of domestic violence by non-Indians in Indian country, as a critical step toward improving public safety on reservations.

“That elevated issues of violence against Native women, which had been going on for decades at intolerably high levels, and put a spotlight on it,” said West, who made many trips to Indian country.

West, whose sister-in-law is California Attorney General Kamala Harris, was born in California. He was publisher of the Harvard Political Review as an undergraduate and was president of the Stanford Law Review. He first served in the Justice Department in the Clinton administration as a special assistant to the deputy attorney general. Before joining the Obama administration, he was a partner at Morrison & Foerster in San Francisco.

West also has dabbled in politics, running unsuccessfully for a state assembly seat in California.

West’s daughter, Meena Harris, 29, is an associate at a Washington law firm, and his wife, Maya Harris, is a senior fellow at the Center for American Progress.

Attorney General Eric H. Holder Jr., in a statement, called West “an exemplary and dedicated public servant.”

“His service as Associate Attorney General has been marked by significant achievement — from his leadership in securing the landmark reauthorization of the Violence Against Women Act; to his passionate advocacy for the rights of American Indian and Alaska Native peoples; to his tireless work to combat financial fraud, hold corporations accountable, and fight for American consumers,” Holder said.