"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,' " Collins wrote. "If I had my way, someone else would have already done this. Nobody has, which is why I'm raising my hand."
Saying he had "endured years of misery and gone to enormous lengths to live a lie," Collins immediately drew support for his announcement from the White House, former President Bill Clinton, the NBA, current and former teammates, a sponsor, and athletes in other sports.
Los Angeles Lakers star Kobe Bryant said in a Twitter post that he was proud of Collins, writing: "Don't suffocate who u r because of the ignorance of others," followed by the words "courage" and "support."
"We've got to get rid of the shame. That's the main thing. And Jason's going to help that. He's going to help give people courage to come out," said Billie Jean King, a member of the International Tennis Hall of Fame who confirmed she was gay after being outed in the early 1980s. King is the owner of the Philadelphia Freedoms of World Team Tennis.
"I guarantee you he's going to feel much lighter, much freer. The truth does set you free, there's no question. It doesn't mean it's easy. But it sets you free," King said in a telephone interview with the Associated Press.
The Wizards, whose season ended April 17, issued a statement from president Ernie Grunfeld: "We are extremely proud of Jason and support his decision to live his life proudly and openly. He has been a leader on and off the court and an outstanding teammate throughout his NBA career."
Collins' coach with the Celtics, Doc Rivers, drew a comparison between Monday's announcement and Jackie Robinson's role when he joined the Brooklyn Dodgers in 1947, breaking the color barrier in Major League Baseball.
"I am extremely happy and proud of Jason Collins. He's a pro's pro. He is the consummate professional and he is one of my favorite 'team' players I have ever coached," Rivers said. "If you have learned anything from Jackie Robinson, it is that teammates are always the first to accept. It will be society who has to learn tolerance."
Collins says he quietly made a statement for gay rights even while keeping his sexual orientation a secret. He wore No. 98 with the Celtics and Wizards - 1998 was year that Matthew Shepard, a gay college student in Wyoming, was killed, and the Trevor Project, a suicide prevention organization, was founded.
According to the General Social Survey, the public has grown increasingly accepting of gay relationships since the late 1980s. That survey found in 1987 that 76 percent of Americans thought sexual relations between adults of the same sex were morally wrong. That fell to 43 percent by 2012.
While some gay athletes have talked in the past about concerns that coming out would hurt their earning potential, 12-time Grand Slam singles champion King said she thinks Collins' openness could have the opposite effect.
"I have a feeling he's got a whole new career," King said. "I have a feeling he's going to make more in endorsements than he's ever made in his life."
Sports equipment maker Nike released a statement Monday saying: "We admire Jason's courage and are proud that he is a Nike athlete. Nike believes in a level playing field where an athlete's sexual orientation is not a consideration."
Momentum has been building toward this sort of announcement from a pro athlete in a top league in the United States. NFL players Brendan Ayanbadejo and Chris Kluwe were outspoken in support of state gay-marriage amendments during last year's elections. President Obama spoke about his support for gay marriage during his reelection campaign.
On Monday, NFL commissioner Roger Goodell sent a memo to teams reiterating the league's antidiscrimination policy about sexuality. It includes a section on questions teams cannot ask prospective draft picks and free agents. After the NFL combine in February, three players said officials posed questions about sexual orientation.
Earlier this month, the NHL and its players' union partnered with an advocacy organization fighting homophobia in sports, and commissioner Gary Bettman said the You Can Play Project underlines that "the official policy of the NHL is one of inclusion on the ice, in our locker rooms, and in the stands."
A first-round draft pick in 2001, Collins has averaged 3.6 points and 3.8 rebounds for the New Jersey Nets, Memphis Grizzlies, Minnesota Timberwolves, Atlanta Hawks, Celtics, and Wizards.
Never a star, he acknowledges, "I take charges and I foul - that's been my forte. ... I set picks with my 7-foot, 255-pound body to get guys like Jason Kidd, John Wall, and Paul Pierce open. I sacrifice myself for other players."
He continues: "I go against the gay stereotype, which is why I think a lot of players will be shocked: That guy is gay? But I've always been an aggressive player, even in high school. Am I so physical to prove that being gay doesn't make you soft? Who knows? That's something for a psychologist to unravel."
Collins says that if he remains in the NBA, he could face uncomfortable reactions from spectators.
"I don't mind if they heckle me. I've been booed before. There have been times when I've wanted to boo myself. But a lot of ill feelings can be cured by winning," he writes.
He adds: "I hope fans will respect me for raising my hand. And I hope teammates will remember that I've never been an in-your-face kind of guy. All you need to know is that I'm single. I see no need to delve into specifics."