US Olympic champion turned reality TV star Bruce Jenner has come out as transgender, saying: "For all intents and purposes, I'm a woman."

In a highly-anticipated TV interview, the 65-year-old said he had wrestled with his sexual identity since he was small.

"I've always been confused about my gender identity since I was this big," he told US network ABC's celebrity interviewer Diane Sawyer.

"Here I am, stuck - and I hate the word - (a) girl stuck in a guy's body. I hate that terminology."

Asked point blank if he was a woman, Jenner said: "Yes. For all intents and purposes, I'm a woman. People look at me differently."

Just before making the long-expected announcement Jenner symbolically let his hair down, physically.

"Let's take the damn ponytail out," he told Sawyer, smiling.

"As of now I have all the male parts and all that kind of stuff," he said.

But he added: "My heart and my soul and everything that I do in life, it is part of me, that female side is part of me, that is who I am," even though "I was not genetically born that way."

"Bruce lives a lie. She is not a lie," Jenner said.

Speculation that the actor, race-car enthusiast and 1976 Montreal Summer Olympics decathlon gold medallist was undergoing a sex change has been rich fodder for supermarket tabloids for months.

Supposed telltale signs, based on paparazzi photos from the streets of Los Angeles, include shaved limbs, long hair worn in a ponytail and what appears to be a sports bra under a T-shirt.

Born outside New York in October 1949, thrice-married Jenner became an American sports hero when he set a world record with his Montreal decathlon victory.

Leveraging his fame, he appeared on boxes of Wheaties, a popular American cereal known as "the breakfast of champions", then tried his hand as a movie actor, with less than stellar results.

But it was through his marriage to the former Kris Kardashian that he once again became a household name in Keeping Up with the Kardashians in which he appeared as the sometimes bemused stepfather of her flamboyant daughters Kourtney, Kim and Khloe.

He and Kris Kardashian have two daughters of their own, but they obtained a divorce in December, 14 months after separating - although he has been seen still wearing a wedding ring.

Lesbian, gay bisexual and transgender (LTBT) rights group GLAAD welcomed Jenner's announcement.

"Today, millions of people learned that someone they know is transgender," said GLAAD head Sarah Kate Ellis.

"By sharing this story, Bruce Jenner has shined a light on what it means to be transgender and live authentically in the face of unimaginable public scrutiny.

"Though Jenner's journey is one that is deeply personal, it is also one that will impact and inspire countless people around the world," she added.

Jenner's son Brandon tweeted simply: "Proud son", along with an Instagram picture of himself as a young boy perched on his smiling father's shoulders.