The allegation, made without evidence, comes as Democratic rival Hillary Clinton's lead in the popular vote over Trump has surpassed 2 million votes and is expected to grow to more than 2.5 million as ballots in populous states such as California continue to be tallied.
Clinton's legal team said on Saturday it had agreed to participate in a recount of Wisconsin votes after the state's election board approved the effort requested by Green Party candidate Jill Stein, which Trump has called "ridiculous."
"In addition to winning the Electoral College in a landslide, I won the popular vote if you deduct the millions of people who voted illegally," Trump tweeted as reporters waited for him to leave his Mar-a-Lago golf resort in Florida to fly back to his residence in New York City.
Before the election, Trump made unsubstantiated allegations that the results of the election might be "rigged" against him.
Since the vote, Trump's message has alternated between appealing for unity and railing against his opponents and the media.
