He made the unprecedented assertion in response to accusations of unwanted kissing, groping and more from at least eight women.
Trump maintained his innocence and denounced Clinton and a supposed international media conspiracy aimed at denying him the White House.
"100 percent fabricated and made-up charges, pushed strongly by the media and the Clinton Campaign, may poison the minds of the American Voter. FIX!,” Trump tweeted on Saturday.
"This election is being rigged by the media pushing false and unsubstantiated charges, and outright lies, in order to elect Crooked Hillary!" he added, the Independent reported.
He went on to say Clinton should be in jail and repeated his assertion the election looks to have been rigged.
Trump called the women who accused him of sexual assault "sick," in a speech at a rally in Greensboro, North Carolina.
The Republican nominee said he had never met most of the women who had made "phony" claims about him.
"I have no idea who these women are," he told the crowd. "I have no idea. I have no idea. And I think you know I have no idea as you understand me for a lot of years, OK?"
"These people are sick," he added.
Polls suggest Trump has fallen further behind Clinton nationally and in most battleground states.
Early in-person voting is underway in 20 states, including Ohio, where US President Barack Obama railed against Trump a day after first lady Michelle Obama declared in a passionate speech, "Enough is enough."
The Obamas both seized on Trump's words, captured in a video released last week, bragging about kissing and groping women. The 70-year-old billionaire has apologized, but also repeatedly dismissed his comments as "locker room talk."
"You don't have to be a husband or a father to know that that kind of language, those kinds of thoughts, those kinds of actions are unacceptable. They're not right. You just have to be a decent human being,"Obama charged in Columbus, Ohio.