A Facebook post making the same accusation has also had a warning attached by that social media platform.
Trump’s tweet said: “We are up BIG, but they are trying to STEAL the Election. We will never let them do it. Votes cannot be cast after the Polls are closed!”
His tweet came with no winner declared in the race for the White House yet, and the US facing the possibility of not hearing who their president will be for some time.
Watch: The latest from the US election
Trump’s message followed Joe Biden’s speech to supporters, in which he said: “I’m here to tell you tonight, we believe we’re on track to win this election.”
The president’s original tweet was ridiculed and he later deleted it, having spelt “polls” as “Poles”.
Trump has tried to drum up controversy about mail in ballots, which have seen wider use due to people’s concerns about voting in person during a pandemic.
He falsely said they have “big problems and discrepancies” and there had been speculation he would call the election’s integrity into question.
Twitter has added a warning to his message, stating: “Some or all of the content shared in this Tweet is disputed and might be misleading about an election or other civic process.”
The company’s Twitter Safety account said it happened “in line with our Civic Integrity Policy”.
Facebook’s label on his post said: “Final results may be different from initial vote counts, as ballot counting will continue for days or weeks.”
Trump’s campaign hit back, saying: “Silicon Valley continues its campaign to censor and silence the president.”
Votes in several key states, including Pennsylvania, Michigan and Wisconsin, have yet to be fully counted, but the election is expected to be close.