McKayla Maroney says she was sexually abused by former USA gymnastics doctor Larry Nassar.
McKayla Maroney, 21, said she was abused over a seven-year period by Nassar.
Nassar is in prison awaiting trial in Michigan on multiple criminal sexual conduct charges and earlier this year admitted possessing child abuse images.
Maroney said the “unnecessary and disgusting” abuse started at a national team training camp in Texas.
“It didn’t end until I left the sport,” she said.
But she was inspired to go public with her claims on Wednesday by others speaking out in the wake of separate allegations against Hollywood film producer Harvey Weinstein.
Maroney won team gold as part of the US gymnastics squad at the London 2012 Games, and took individual silver on the vault.
“I was molested by Dr Larry Nassar, the team doctor for the US women’s national gymnastics team, and Olympic team,” she wrote on Twitter.
“It seemed whenever and wherever this man could find the chance, I was ‘treated’. It happened in London before my team and I won the gold medal, and it happened before I won my silver.”
Her statement detailed other occasions she was allegedly abused by Nassar. She is the most high-profile athlete to detail alleged abuse by Nassar.
Hundreds of women and girls have also come forward to say they were sexually assaulted by him, according to the Indianapolis Star newspaper, which led an investigation into Nassar.
The doctor, who is in his fifties, spent almost three decades working with USA Gymnastics and was involved with the team at four Olympic Games. He has denied the abuse claims.
USA Gymnastics president and chief executive chief Steve Penny resigned over the issue.
The organisation said it “admires the courage of those, like McKayla Maroney, who have come forward to share their personal experiences with sexual abuse.
“We are outraged and disgusted by the conduct of which Larry Nassar is accused. We are sorry that any athlete has been harmed during her or his gymnastics career.”
It said it was “strengthening and enhancing our policies and procedures regarding abuse”.