After months of speculation, Bruce Jenner has said he is transgender and identifies as a woman.
"For all intents and purposes, I am a woman," he told Diane Sawyer on ABC.
Jenner was speaking in a two-hour interview on ABC News where he confirmed that he'd been wrestling with his gender since childhood.
The US Olympic gold medallist, who won the decathlon at the Montreal games in 1976, is seen as the highest-profile American to come out as transgender.
"That female side of me is part of me. It's who I am. I was not genetically born that way.
"As of now I have all of the male parts. So in a lot of ways we are different," he added.
Jenner was asked if it was a publicity stunt to promote his reality show, but he laughed at this and denied it.
Entertainment channel E! has said a new documentary series will follow Bruce's life as a transgender woman, starting in July.
The reality TV star said his stepdaughter, Kim Kardashian, had been a big supporter and so had her husband Kanye West.
Jenner said that the past few months had been difficult but he was going to be fine.
"Yeah, I hope I'm gonna be OK. I feel like I'm gonna be OK. 2015's gonna be quite a ride... quite a ride."
He added: "People look at me differently, they see you as this macho male but my heart and my soul and everything I do in life it is part of me."
He admitted that it had been something he has lived with all his life. He said that when he was eight he started dressing up in his mother and sister's clothes.
"At that time I didn't know why I was just doing it, just that it felt good."
He denied speculation that he is involved in a publicity stunt and said he hoped his story could help other transgender people.
"What I am doing is going to do some good. We're gonna change the world. I really, firmly believe that.
"We're going to make a difference in the world with what we're doing."
As Bruce Jenner has not requested that a new name or pronoun be used, Newsbeat has referred to him by his current name, and the male pronoun throughout in order to respect his wishes.