The 255-pound Sam played for Hitchcock High School and graduated in 2009.
In interviews with ESPN, The New York Times and Outsports that were published Sunday, Sam said his teammates and coaches at Missouri have known since August.
"I am an openly, proud gay man," he said.
He participated in the Senior Bowl in Mobile, Ala., last month after leading the SEC in sacks (11.5) and tackles for loss (19), and he is projected to be a mid-round NFL draft pick.
"It's a big deal. No one has done this before. And it's kind of a nervous process, but I know what I want to be ... I want to be a football player in the NFL," he said in the interviews.
There have been numerous NFL players who have come out after their playing days, including Kwame Harris and Dave Kopay.
Last year, NBA player Jason Collins announced he was gay after the season. Collins was a free agent and has not signed with a new team this season. MLS star and U.S. national team player Robbie Rogers also came out a year ago.
Division III Willamette kicker Conner Mertens, a redshirt freshman, said last month he was bisexual.
"We admire Michael Sam's honesty and courage," the NFL said in statement. "Michael is a football player. Any player with ability and determination can succeed in the NFL. We look forward to welcoming and supporting Michael Sam in 2014."
Sam said many people at the college all-star game for NFL prospects seemed to know that he was gay.
"I didn't realize how many people actually knew, and I was afraid that someone would tell or leak something out about me," he told ESPN. "I want to own my truth. ... No one else should tell my story but me."
Before coming out to all his teammates and coaches, Sam said he told a few close friends and dated another Missouri athlete who was not a football player.
"Coaches just wanted to know a little about ourselves, our majors, where we're from, and something that no one knows about you," Sam said. "And I used that opportunity just to tell them that I was gay. And their reaction was like, 'Michael Sam finally told us.'"
Missouri coach Gary Pinkel said in a statement Sunday night he was proud of Sam and how he represented the program.
"Michael is a great example of just how important it is to be respectful of others, he's taught a lot of people here first-hand that it doesn't matter what your background is, or your personal orientation, we're all on the same team and we all support each other," Pinkel said. "If Michael doesn't have the support of his teammates like he did this past year, I don't think there's any way he has the type of season he put together."
Missouri linebacker Donovan Bonner has been a teammate of Sam's for five years.
"We knew of his status for 5 years and not one team member, coach, or staff member said anything says a lot about our family atmosphere," Bonner tweeted.
The Associated Press contributed to this story.
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