The five-times Olympic gold medalist made the admission to British journalist Michael Parkinson in a pre-recorded interview on Australia's Channel 10 in which he shared his relief in freeing himself from living "a lie."
"I've thought about this for a long time. I'm not straight," the 31-year-old said, struggling to hold tears back.
"And this is only something that very recently, we're talking the past two weeks, I've been comfortable telling the closest people around me, exactly that.
"I've wanted to (come out) for some time but I couldn't, I didn't feel as though I could.
"What happened was I felt the lie had become so big that I didn't want people to question my integrity."
One of Australia's favorite sons, Thorpe had long denied he was gay and wrote in his 2012 autobiography "This Is Me" that he was heterosexual.
"For the record, I am not gay and all my sexual experiences have been straight," Thorpe wrote in the book. "I'm attracted to women, I love children and aspire to have a family one day."
Smartly dressed in a navy blue coat and sporting a face of stubble, Thorpe said he had long battled to accept his sexuality, and might have come out earlier if he had not been constantly questioned about it from the age of 16.
"The problem was I was asked at such a young age about my sexuality. I went to an all boys school ... so if you're accused of being gay, the first answer is no and you get ready for a fight," he said.
Being called a homophobic slur and other taunts by members of the public had also not helped.
However, he admitted a big part of his reticence was that he worried that being gay would not fit into his image as "Australia's champion."
"Now it'll be something that I work on with a doctor," he said.
"This trying to live a lie which I was doing. But I was already living somewhat of a lie in my life because I was trying to be what I thought was the right athlete by other people's standards.
Wave of supports
"I wanted to make my family proud, I wanted to make my nation proud. Part of me didn't know if Australia wanted it's champion to be gay.
"People will criticize me, some people won't like the idea other people may applaud me for it, but it's me."
Thorpe's revelations sparked a wave of support from fellow athletes, celebrities and gay activists on social media.
"I can totally understand how difficult this whole process has been for him," Australian Olympian Matthew Mitcham, who came out before winning a diving gold at the 2008 Beijing Games told Sydney newspaper, the Telegraph.
"I really hope this process gives him some peace and that the media and the public give him the same respect and the same overwhelming support I received in 2008."