(CNN) -- It's the biggest move of his career and it's off the court.
Jason Collins, who played with the NBA's Boston Celtics and Washington Wizards this season, has disclosed that he is gay, making him the first active openly gay male athlete in the four major American pro team sports.
The center, who said he is now a free agent, made the disclosure in a column appearing in the upcoming issue of Sports Illustrated.
"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 wrote.
Collins also wrote that the killing of three people at the Boston Marathon persuaded him to make an announcement. Things can change in a moment, so why not start living more truthfully, he wrote.
On Twitter, Collins thanked everyone who sent him messages of support.
"All the support I have received today is truly inspirational. I knew that I was choosing the road less traveled but I'm not walking it alone," he wrote.
Sports Illustrated's managing editor said Collins was reticent about being a flag bearer.
"It is a much simpler, more personal reason. He wants to have a family," Chris Stone told CNN. "He wants the same life that his twin brother, Jarron, has. This is a secret he's kept for a very long time. He didn't even inform his brother that he was gay until late last summer. ... So much for twin telepathy."
Jarron, who played in the NBA for 10 seasons and was Jason's teammate at Stanford, tweeted to his brother: "Very proud of you."
"Jason Collins has forever changed the face of sports," said the Human Rights Campaign, a civil rights group fighting for gay rights.
It likened the announcement to Jackie Robinson, the first African-American to play in major league baseball in the modern era.
"At a time when millions are reflecting on the life and legacy of Jackie Robinson, Jason Collins is a hero for our own times," the group said.
Former NBA player Charles Barkley, now an analyst for "Inside the NBA" on TNT, told CNN on Monday night that active players shouldn't be surprised that a teammate is gay.
"I think anybody who thinks they never played with a gay player is an idiot," he said on "AC360." "I played with several gay players. It's their own business, and I think they should get to be who they want to be."
Collins is a 7-footer who has played with six NBA teams -- the Wizards, Celtics, Atlanta Hawks, Minnesota Timberwolves, Memphis Grizzlies and New Jersey Nets -- over the past 12 seasons. He has averaged 3.6 points and 3.8 rebounds in 713 career games, and he has had a great deal of playoff experience with the Nets (who have since moved to Brooklyn) and the Hawks.
Momentum had been growing in recent months for an active player in the National Football League, National Hockey League, National Basketball Association or Major League Baseball to come out.
Just a few weeks ago, the NHL announced a new program teaching tolerance and giving support to gay athletes.
Commissioner Gary Bettman said the agreement made the NHL's policy of inclusiveness "clear and unequivocal."
"While we believe that our actions in the past have shown our support for the LGBT community, we are delighted to reaffirm through this joint venture with the NHL Players' Association that the official policy of the NHL is one of inclusion on the ice, in our locker rooms and in the stands," Bettman said in the joint statement with You Can Play.
And NFL player Brendon Ayanbadejo, The New York Times reported, has been in contact with closeted players in several sports.
"What we're trying to facilitate is to get them together and do what they want to do, do what is right for them," he told the Times.
In February, former professional soccer player Robbie Rogers announced on his blog that he is gay. His former U.S. teammates showed solidarity in response to the news.
Though reaction to Collins' announcement also was overwhelming support, not everyone had a positive response.
ESPN basketball analyst Chris Broussard said the Bible calls homosexuality a sin.
"If you're openly living in unrepentant sin, whatever it may be, not just homosexuality, adultery, fornication, premarital sex between heterosexuals, whatever it may be. I think that's walking in open rebellion to God and to Jesus Christ," he said.
NFL receiver Mike Wallace tweeted Monday: "All these beautiful women in the world and guys wanna mess with other guys SMH."
He tweeted again, saying he was being misinterpreted.
"Never said anything was right or wrong I just said I don't understand!! Deeply sorry for anyone that I offended," he wrote.
The Dolphins, Wallace's employer, issued a statement saying the team has spoken with Wallace about the tweets. The team said it will address its policy of inclusion with all of its players.
NBA Commissioner David Stern said Collins, a 34-year-old veteran, is widely respected in the league.
"We are proud he has assumed the leadership mantle on this very important issue," he said in a statement.
Washington Wizards President Ernie Grunfeld said the team is "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 orientations," Grunfeld said.
Boston Celtics head coach Doc Rivers called Collins "a pro's pro" and one of his favorite players he ever coached.
"If you have learned anything from Jackie Robinson, it is that teammates are always the first to accept," Rivers said. "It will be society who has to learn tolerance."
Bill Clinton called the announcement an "important moment for professional sports and in the history of the LGBT community."
Collins, who had been traded by the Celtics to the Wizards this season, wrote in his essay that U.S. Rep. Joe Kennedy, D-Massachusetts, helped prompt his decision to come out as gay. Kennedy and Collins were roommates at Stanford. He recounts hearing about Kennedy, who is not gay, marching in Boston's Gay Pride Parade.
"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," Kennedy said. "I'm proud to stand with him today and proud to call him a friend."
President Barack Obama called Collins "to express his support and said he was impressed by his courage," a White House official said.
Active players posted their approval on Twitter. The Los Angeles Lakers' Kobe Bryant said: "Proud of @jasoncollins34. Don't suffocate who u r because of the ignorance of others." Steve Nash, also a Laker, said: "The time has come. Maximum respect."
Two Wizards players, Garrett Temple and Bradley Beal, said in tweets they were "proud" of Collins. "Much respect to my teammate," Temple said. "Great teammate, mentor and better person," said Beal.
Collins' announcement comes after the end of the Wizards' season, and he is no longer under contract. He is an "old" player by NBA standards who has never been a star. So will he play in the NBA this fall?
ESPN.com said only six of 14 team officials reached for an unofficial poll Monday thought he would be back in the league. If he does return, he probably will play only a few minutes a game for a team that needs a veteran leader, the other executives told ESPN.com.
Some male athletes have come out as gay after they've left professional sports. One is John Amaechi, a former NBA player.
Amaechi told CNN he hopes Collins will be a catalyst for a wider acceptance of openly gay athletes, saying he believes Collins is better equipped than anyone who came before him to handle the attention that will come his way. But it may take more, Amaechi said.
People like to believe one iconic figure can change things, he said, "but the reality is that when there's this tipping point, or enough people coming together deciding that change is necessary, that's when change happens."
The Women's National Basketball Association, the women's pro league, has had its share of milestones and openness as far as gays are concerned. In 2005, Sheryl Swoopes, a top player in the WNBA, announced she was gay.
Brittney Griner, the No. 1 pick in this year's WNBA draft, recently said in an interview with Sports Illustrated that she is gay. Griner finished her college career as the NCAA's all-time leading shot-blocker with 748 and as the No. 2 all-time scorer with 3,283 points.
CNN's Brianna Keilar, Jason Hanna and Jill Martin contributed to this report.