He told a rally in Columbus, Ohio, that he had heard "more and more" that the contest would be unfair. He offered no immediate evidence.
On the forthcoming vote, he told supporters "I'm afraid the election is going to be rigged, I have to be honest".
He later repeated the claim on Fox News, adding "I hope the Republicans are watching closely or it's going to be taken away from us."
Trump has made similar comments before in relation to the Democratic race, suggesting the party fixed its system to favour nominee Hillary Clinton over her challenger Bernie Sanders.
Earlier this year, he also complained the Republican primary system was also "rigged" amid party efforts to stop his march to the candidacy.
Correspondents say Trump could be trying to capitalise on voters' distrust of establishment institutions, or lay the groundwork should he lose the election.
Trump has come under fire from across the political divide for remarks he made about the parents of a US Muslim soldier killed in action.
At another rally in Pennsylvania, he took the unprecedented step of directly calling Mrs Clinton "the devil".
He attacked Mr Sanders for capitulating in the Democratic race, saying he "made a deal with the devil. She's the devil."
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