People magazine, a trusted conduit for Hollywood publicists, reported this week that Jenner, 65, would open up in a forthcoming interview with ABC News.
"Bruce is transitioning to a woman," the celebrity news weekly quoted a source close to the family as saying, after weeks of tabloid speculation.
"He is finally happy and his family is accepting of what he's doing. He's in such a great space. That's why it's the perfect time to do something like this."
Jenner's 88-year-old mother Esther Jenner, who lives in Idaho, meanwhile told RadarOnline.com that she had "just learned" about her son's gender transition.
"I have never been more proud of Bruce for who he is, himself as a father, as an Olympian, a wonderful public speaker," she told the gossip website.
"He instills enthusiasm in people. He's gifted."
In the latest twist, the Kardashian family -- into which Jenner has been married -- cancelled all press events Thursday in the run-up to season 10 of their hit reality TV show "Keeping Up with the Kardashians."
Jenner himself has been lying low, ahead of the premiere of a documentary that will reportedly chronicle his gender transition.
Speculation that the actor, race-car enthusiast and 1976 Montreal Summer Olympics decathlon gold medallist is 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.
Throat surgery clue
In late January, Jenner was seen emerging from a Beverly Hills clinic with a bandage around neck, presumably after surgery to flatten his Adam's apple.
Known as a laryngeal shave, the procedure -- which involves reducing cartilage in the throat for a more feminine look -- is common among transgenders.
"I just never liked my trachea," he told the TMZ.com gossip website prior to the outpatient operation.
American society is widely perceived as becoming more accepting of gender diversity, as seen by the legalization of same-sex marriage in many states.
Transgender people, however, still face "staggering levels of discrimination and violence," according to LGBT media activist group GLAAD.
In the media, they typically appear as either victims or villains, GLAAD said, although the Golden Globe-winning Amazon original series "Transparent" has cast them in a more sympathetic light.
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.
He did better on television, appearing in the 1980s police series "CHiPs" and sitcom "Silver Spoons" among other shows, and in 2003 the American version of "I'm a Celebrity... Get Me Out of Here!"
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.
"I had it all, and then I met Kris," he quipped in one episode.
"It's typical for me to be forgotten in this family," he added in another.
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.
