Mr Trump continues to deny numerous claims from women who allege he made unwanted sexual advances towards them.
The latest allegation comes as he again is refusing to commit to accepting the result of November's presidential election.
A New York yoga teacher has come forward saying she was the victim of a sexual assault by United States Republican presidential nominee Donald Trump back in 1998.
She becomes the 10th woman to accuse the billionaire businessman of unwanted sexual advances.
Karena Virginia says the incident happened in 1998 as she waited for a car outside the US Open tennis tournament in the Flushing neighbourhood of New York City.
Mr Trump allegedly walked up to her, grabbed her right arm and touched her breast while saying to her, "Don't you know who I am?"
"Your random moment of sexual pleasure came at my expense and affected me greatly. Mr Trump revealed his true character in his own words on tape, which indicated that he felt entitled to grab women by their private parts. We should, to paraphrase Maya Angelou, believe him ... to paraphrase Maya Angelou, believe him."
Ms Virginia says, for years afterward, she felt shame and disgust over the incident, until she realised she was the victim and not to blame.
Mr Trump has continually denied allegations of sexual assault, calling all the stories of sexual misdeeds totally false.
He added during the last debate, "Nobody has more respect for women than I do."
The woman's lawyer, Gloria Allred, questions those comments.
"His response is ludicrous in light of the fact that women have come forward with allegations that Mr Trump engaged in kissing them without their consent, groping them, grabbing their breasts and vaginas, forcing his tongue down one woman's throat, attempting to have a sexual relationship with another, and generally behaving in accordance with Mr Trump's own words, which were captured on tape, bragging about what he wanted to do with women and to women."
The latest allegation comes as Mr Trump says he will reserve the right to file a legal challenge in the case of a "questionable result" in the upcoming election.
His refusal to commit to accepting the results of the election have alarmed both Democrats and Republicans, but he says he remains optimistic of his chances of winning.
"Of course, I would accept a clear election result. But I would also reserve my right to contest or file a legal challenge in the case of a questionable result. And always, I will follow and abide by all of the rules and traditions of all of the many candidates who have come before me. Always. Bottom line, we're going to win. We're going to win. We're going to win so big. We're going to win so big. November 8th, we're going to win."
Mr Trump claims the election is being rigged against him, and that has become the latest flashpoint in an unusually volatile race three weeks before voters go to the polls.
With him trailing in opinion polls, the focus ahead of the November 8 vote is shifting to Congress.
The question has become whether Republicans will keep their narrow majority in the Senate or even their larger advantage in the House of Representatives.
President Barack Obama has accused Mr Trump of what he calls "sowing the seeds of doubt" about American democracy.
Mr Trump's national political director has reportedly decided to take what he calls a "step back" from his candidate's campaign.
And First Lady Michelle Obama has again spoken out against the Republican candidate, saying he is threatening what she calls "the very idea of America."
"We are fortunate to live in a country where the voters decide our elections, the voters decide who wins and loses, period, end of story. And when a presidential candidate threatens to ignore our voices and reject the outcome of this election, he is threatening the very idea of America itself. And we cannot stand for that. You do not keep American democracy in suspense, because, look, too many people marched and protested and fought and died for this democracy."
