Caitlyn Jenner, candidate for California governor and former Olympic gold medallist, says she opposes trans girls participating in women's sports.
Ms Jenner, who came out as a trans woman in 2015, told a reporter: "It just isn't fair. And we have to protect girls' sports in our schools."
The Republican candidate for governor gave what appeared to be an impromptu interview to the TMZ website.
A number of US states are considering a ban on trans girls in women's sports.
Mississippi signed such a ban into law in March, although it is expected to face appeals. The Human Rights Campaign - the largest LGBT advocacy group in the US - says some 17 other states are considering similar legislation.
I didn’t expect to get asked this on my Saturday morning coffee run, but I’m clear about where I stand. It’s an issue of fairness and we need to protect girls’ sports in our schools.https://t.co/YODLDQ3csP— Caitlyn Jenner (@Caitlyn_Jenner) May 1, 2021
Caitlyn Jenner was one of the US's most successful athletes in the decathlon during the 1970s and won gold in the Montreal Olympics in 1976.
In recent years, she has been a household name thanks to her involvement in the hit reality show Keeping Up with the Kardashians. Before transitioning, she was married to Kris Jenner and the pair have two daughters, Kendall and Kylie.
Ms Jenner, who has been described as the highest-profile American to come out as transgender, was asked for her opinion on the hot-button issue of trans athletes while out walking her dog to get a coffee.
"This is a question of fairness," she said. "That's why I oppose biological boys who are trans competing in girls' sports in school. It just isn't fair. And we have to protect girls' sports in our schools."
Ms Jenner then took to Twitter to reiterate her stance.
The 71-year-old announced her bid to run as a Republican in the Democratic stronghold state of California two weeks ago.
Confirmation of a recall election is expected after a petition against current Governor Gavin Newsom, a Democrat, reached the number required to trigger a vote. Voters would be asked if they want Mr Newsom to stay or another candidate to take on the job.
Ms Jenner's views on trans athletes put her at odds with many activists in the trans community, who argue that legislation targeting trans children is dangerous and discriminatory.
On Friday, another transgender celebrity, Elliot Page, criticised the moves to ban transgender athletes from competing in girls' sports teams in some states.