US basketball player Jason Collins has come out as gay, the first active male athlete in a major American professional team sport to do so.
He declared his sexuality in an article for Sports Illustrated, announcing: "I'm a 34-year-old NBA center. I'm black. And I'm gay."
President Barack Obama, NBA star Kobe Bryant and sportswear company Nike were among those expressing public support.
Former NBA player John Amaechi came out in 2007, but had already retired.
On Monday, Mr Obama, who last year announced his support for gay marriage, called Collins to tell him he was impressed with his courage and offer his support, the White House said.
Former President Bill Clinton called the move "an important moment for professional sports and in the history of the LGBT community".
His daughter, Chelsea, who knew Collins when they were both students at California's Stanford University, tweeted: "Very proud of my friend Jason Collins for having the strength and courage to be the first openly gay player in the NBA."
NBA Commissioner David Stern said in a statement: "Jason has been a widely respected player and teammate throughout his career and we are proud he has assumed the leadership mantle on this very important issue."
Los Angeles Lakers guard Kobe Bryant was among several of Collins's fellow NBA players to offer his support publicly.
"Don't suffocate who u r because of the ignorance of others," Bryant tweeted, followed by the words "courage" and "support".
Sportswear company Nike, which has endorsed Collins, also supported his decision.
"Jason is a Nike athlete," its statement said. "We are a company committed to diversity and inclusion."
The reaction was not entirely welcoming: An analyst for sports broadcaster ESPN, Chris Broussard, said on Monday he did not believe that "you can live an openly homosexual lifestyle" and be a Christian. Collins had written in the article that he took "the teachings of Jesus seriously".
Symbolic shirt number
In the Sports Illustrated article, Collins, who has most recently played for the Washington Wizards and the Boston Celtics, said: "I didn't set out to be the first openly gay athlete playing in a major American team sport.
"But since I am, I'm happy to start the conversation. I wish I wasn't the kid in the classroom raising his hand and saying, 'I'm different.' If I had my way, someone else would have already done this. Nobody has, which is why I'm raising my hand."
He added that this month's bombings at the Boston Marathon had reinforced his conclusion that he should talk publicly about his sexuality.
"Things can change in an instant, so why not live truthfully?" Collins wrote.
Collins, who has played 11 seasons in the NBA with six teams and is not currently attached to a team, said he had tried to suppress his feelings through relationships with women.
"When I was younger I dated women. I even got engaged," he said. "I thought I had to live a certain way.
"I thought I needed to marry a woman and raise kids with her. I kept telling myself the sky was red, but I always knew it was blue."
Collins said he decided he should go public after his former roommate at Stanford University, Congressman Joe Kennedy, scion of the Kennedy political dynasty, marched in a Boston gay pride parade.
As basketball player at Stanford, Collins competed in the national collegiate championship tournament, reaching the fourth round. He has also played in two NBA finals. His twin brother, Jarron, is a former NBA player.
Collins explained that in 2012 he changed his uniform number to 98 - a number with significance for the gay community in the US - as a gesture of solidarity.
In 1998, Matthew Shepard, a gay student at the University of Wyoming, was brutally killed in what has been cited as one of the most notorious anti-gay hate crimes in the US.
Several male athletes have previously come out after retirement, including Amaechi, the NFL's Esera Tuaolo and Major League Baseball's Billy Bean. Collins is the first to do so while active in sport.