His admission on Wednesday came three years after his playing career ended, making him the sixth professional male athlete from one of the four major U.S. team sports — basketball, baseball, football and hockey — to openly discuss his homosexuality.
Amaechi details his life in his autobiography "Man in the Middle," which will be released on Feb. 14.
"He is coming out of the closet as a gay man," said Amaechi's publicist, Howard Bragman.
Martina Navratilova, perhaps the most famous openly gay athlete in the world, praised Amaechi's decision and said it's imperative for athletes to come out because of what she called an epidemic of suicides among young lesbians and gays.
"It's hugely important for the kids so they don't feel alone in the world. We're role models," she said. "He will definitely help a lot of kids growing up to feel better about themselves."
Orlando's Grant Hill, who said he didn't know Amaechi when he was with the Magic, also applauded the decision to go public.
"The fact that John has done this, maybe it will give others the comfort or confidence to come out as well, whether they are playing or retiring," Hill said.
LeBron James, however, said he didn't think an openly gay person could survive in the league.
"With teammates you have to be trustworthy, and if you're gay and you're not admitting that you are, then you are not trustworthy," James said. "So that's like the No. 1 thing as teammates — we all trust each other. You've heard of the in-room, locker room code. What happens in the locker room stays in there. It's a trust factor, honestly. A big trust factor."
Injured Philadelphia 76ers forward Shavlik Randolph acknowledged it's a new situation.
"As long as you don't bring your gayness on me, I'm fine," Randolph said. "As far as business-wise, I'm sure I could play with him. But I think it would create a little awkwardness in the locker room."