A woman has told the Washington Post that Donald Trump reached up her skirt and groped her in a New York City nightspot in the 1990s.
Kristin Anderson said the property mogul touched her through her underwear during the alleged assault.
Ms Anderson, now 46, said she was “very grossed out and weirded out”.
It is the latest claim of sexual misconduct to emerge in recent days against the Republican nominee, who has branded his accusers “liars”.
Responding to this specific accusation, his spokeswoman said Ms Anderson was seeking publicity by making a “phony” complaint.
Ms Allred said Summer Zervos, a former contestant on season five of The Apprentice, was targeted by Mr Trump when she met him at the Beverly Hills Hotel to discuss employment opportunities.
“You should be ashamed of yourself,” said the attorney, who has previously represented alleged sexual assault victims of entertainer Bill Cosby.
In the Washington Post report, Ms Anderson said the incident happened at a Manhattan nightspot, which she believes was the China Club, while she was a waitress trying to make it as a model.
She said she was groped by a man sitting on a red velvet couch and turned around to recognise him instantly as the celebrity property mogul.
“He was so distinctive looking,” she told the newspaper, “with the hair and the eyebrows. I mean, nobody else has those eyebrows.”
She added: “It wasn’t a sexual come-on. I don’t know why he did it. It was like just to prove that he could do it, and nothing would happen.
“There was zero conversation. We didn’t even really look at each other. It was very random, very nonchalant on his part.”
Mr Trump’s spokeswoman, Hope Hicks, said in a statement emailed to the Washington Post: “Mr Trump strongly denies this phony allegation by someone looking to get some free publicity. It is totally ridiculous.”
The newspaper said it had approached Ms Anderson after learning of her story through a third party, and she had spent several days deciding whether to go public.
“I’ve always kept quiet,” she said. “And why should I keep quiet?
“Actually, all of the women should speak up, and if you’re touched inappropriately, tell somebody and speak up about it.
“Actually go to the authorities and press some charges. It’s not okay.”
Republican vice-presidential candidate Mike Pence said new evidence would emerge on Friday that would undermine the sexual accusations against Mr Trump.
“Before the day is out there will be more evidence publicly that calls into question these allegations,” said the Indiana governor.
He added that the “Clinton political machine” was using slander and lies to deflect from real issues.
In recent days, the Republican ticket has been rocked by allegations from several women that Mr Trump made unwanted sexual advances against them, which he has vociferously denied.
The claims began to emerge after he denied at last Sunday’s presidential debate that he had ever committed sexual assault.
Mr Trump made that denial on national live television as he was asked about a leaked recording from 2005 in which he is heard bragging that he can force himself on women because he is a star.
On Thursday, First Lady Michelle Obama said Mr Trump’s sexually aggressive boasts were “shocking and demeaning” and the language of a sexual predator.
Dozens of Republicans have said they can no longer endorse their party’s nominee.
Polls suggest Mr Trump is losing ground in some of the key battleground states against Democratic presidential candidate Hillary Clinton, with just 24 days until the election.