The US President doubled down on his unfounded claims about voter fraud, claiming "bad things happened".
He insisted he won the election with "legal votes" in the tweet.
His latest outburst came just hours after it was announced Biden was projected to win, after securing the key state of Pennsylvania.
Trump failed to provide any evidence for his claims mail-in ballots and election observers.
He tweeted: "THE OBSERVERS WERE NOT ALLOWED INTO THE COUNTING ROOMS.
"I WON THE ELECTION, GOT 71,000,000 LEGAL VOTES.
"BAD THINGS HAPPENED WHICH OUR OBSERVERS WERE NOT ALLOWED TO SEE.
"NEVER HAPPENED BEFORE. MILLIONS OF MAIL-IN BALLOTS WERE SENT TO PEOPLE WHO NEVER ASKED FOR THEM!"
He added: "71,000,000 Legal Votes. The most EVER for a sitting President!"
Social media giant Twitter quickly added a disclaimer on to his posts, saying, "This claim about election fraud is disputed".
His rant follows days of accusations about voter fraud.
Networks declared Biden was projected to win the election earlier on Saturday.
He secured the win after he managed to flip the vital midwest states of Michigan, Wisconsin and Pennsylvania back from Trump.
Candidates need 270 electoral college votes to win, with each state worth a different number of votes based roughly on its population size.
Current projections from news channel CNN had Biden on 279 by Saturday night, November 7, with Trump behind on 214.
Celebrations have taken place in cities across the US since the projection was announced.
Crowds gathered outside Trump Tower in New York to mock the US President.
Meanwhile, Trump was told he had lost while playing golf, according to reports.
Biden has not responded to the latest claims from his opponent.
In a statement earlier today, he said he was “honoured” to have won and pledged to be a “President for all Americans”.