In a first-person article posted Monday on Sports Illustrated's website, Collins begins: "I'm a 34-year-old NBA center. I'm black. And I'm gay."
Collins has played for six teams in 12 seasons, most recently as a reserve with the Washington Wizards after a midseason trade from the Boston Celtics. He is now a free agent and wants to keep playing in the NBA.
"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 writes. "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, and athletes in other sports.
Los Angeles Lakers star Kobe Bryant tweeted 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.
"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. Those qualities will continue to serve him both as a player and as a positive role model for others of all sexual orientation."
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 as the first black player 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.
Momentum has been building toward this sort of announcement from a pro athlete in a top league. NFL players Brendan Ayanbadejo and Chris Kluwe were outspoken in support of state gay-marriage amendments during last year's elections. President Barack Obama spoke about his support for gay marriage during his re-election campaign.
The topic made waves during Super Bowl week when one player, San Francisco 49ers cornerback Chris Culliver, said he wouldn't welcome a gay member of his team. At the time, Ayanbadejo estimated that at least half of the NFL's players would agree with what Culliver said, at least privately.
On Monday, NFL Commissioner Roger Goodell sent a memo to teams reiterating the league's anti-discrimination 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.
Living in the nation's capital last month while the U.S. Supreme Court heard arguments about same-sex marriage had an effect on Collins.
"Less than three miles from my apartment, nine jurists argued about my happiness and my future. Here was my chance to be heard, and I couldn't say a thing," he writes. "I didn't want to answer questions and draw attention to myself."
After being 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.
"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," NBA commissioner David Stern said in a statement.
Wizards guard Garrett Temple texted to the AP: "I was surprised. I didn't know and I was right next to him in the locker room. It definitely took a lot of courage for him to come out. He was a great teammate."
Former Collins teammate Jerry Stackhouse, now with the Brooklyn Nets, wrote in a text: "I hope Jason is received well by our NBA family. Jason is a friend and a former teammate that I've enjoyed many laughs and conversations with and his sexual orientation won't change that with me. I've already reached out to him personally to show support and will encourage more guys to do the same."
Several male athletes have previously come out after they retired, including the NBA's John Amaechi, the NFL's Esera Tuaolo and Major League Baseball's Billy Bean. Rick Welts, president and chief operating officer of the NBA's Golden State Warriors, is openly gay.
But Collins is the first to do so while planning to keep playing.
"I think he is immensely brave. I think it's a shame in this day and age he has to be immensely brave, but he is," Amaechi told the AP. "He's going to be a remarkable and eloquent spokesperson for what it is to be a decent, authentic human being - never mind just for gay people."
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."
In February, former U.S. soccer national team player Robbie Rogers said he was gay - and retired at the same time. Rogers is just 25, and others have urged him to resume his career.
"I feel a movement coming," he tweeted after word of Collins' news broke.
Female athletes have found more acceptance in coming out; Brittney Griner, a top college basketball player now headed to the WNBA, caused few ripples when she said this month she is a lesbian. Tennis great Martina Navratilova, who came out decades ago, tweeted Monday that Collins is "a brave man."
"1981 was the year for me - 2013 is the year for you," her post added.
White House spokesman Jay Carney called Collins' decision courageous and said the administration views it as another example of progress and evolution in the U.S. as Americans grow more accepting of gay rights and same-sex marriage.
Former President Clinton said: "Jason's announcement today is an important moment for professional sports and in the history of the LGBT community. It is also the straightforward statement of a good man who wants no more than what so many of us seek: to be able to be who we are; to do our work; to build families and to contribute to our communities. For so many members of the LGBT community, these simple goals remain elusive."
Collins attended Stanford with Clinton's daughter Chelsea and played in a Final Four while at the school. His twin brother, Jarron, was also a longtime NBA center. Collins says he told his brother he was gay last summer.
Advocacy organization GLAAD released a statement from Aaron McQuade, the head of its sports program.
"Courage' and 'inspiration' are words that get thrown around a lot in sports, but Jason Collins has given both ideas a brand new context," he said. "We hope that his future team will welcome him, and that fans of the NBA and sports in general will applaud him. We know that the NBA will proudly support him, and that countless young LGBT athletes now have a new hero."
At Stanford, Collins was a college roommate of Rep. Joe Kennedy III, D-Mass. In his account, Collins writes that he realized he needed to go public when the congressman walked in Boston's gay pride parade last year - and Collins decided he couldn't join him.
"For as long as I've known Jason Collins he has been defined by three things: his passion for the sport he loves, his unwavering integrity, and the biggest heart you will ever find. Without question or hesitation, he gives everything he's got to those of us lucky enough to be in his life. I'm proud to stand with him today and proud to call him a friend," Kennedy said in a statement.
In Monday's story, Collins writes that the Boston Marathon bombing on April 15 "reinforced the notion that I shouldn't wait for the circumstances of my coming out to be perfect. Things can change in an instant, so why not live truthfully?"
And now, Collins says, he will be in Boston on June 8, marching alongside Kennedy at the city's 2013 gay rights parade.