"He won because the Election was Rigged," Trump wrote before falsely claiming that no watchers or observers had been allowed.
It was not clear whether the tweet represented a grudging or an accidental concession by Trump that he had lost the election, which he has repeatedly claimed to have won, even after every major news organization projected Biden as the victor.
A White House official, asked whether Trump was admitting that Biden won, said: "It looks like it."
Asked whether the tweet was the beginning of Trump's version of a concession, the official said "it very well may be" and noted that it was the second such signal Trump had sent in recent days. The first was his Rose Garden slip-up, in which he mused about the possibility of another administration's taking over. And he has also seemed to admit that Biden had won Arizona.
But a few minutes after the tweet, Trump appeared to rush back to Twitter to make it clear that he was not conceding.
"He only won in the eyes of the FAKE NEWS MEDIA," Trump wrote. "I concede NOTHING!" He went on to repeat his false claim that the election was rigged.
In his tweets, Trump falsely said mechanical glitches with voting machines on election night "were really THEM getting caught trying to steal votes."
Just before midnight Sunday, Trump reiterated his false claim on Twitter in all caps, "I WON THE ELECTION!"
Top government and industry officials have said that the 2020 election was "the most secure in American history" and that there was "no evidence that any voting system deleted or lost votes, changed votes or was in any way compromised."
Twitter quickly added a warning to Trump's post. The company has added multiple labels to tweets from the @realDonaldTrump account, including many that made unfounded allegations of voting fraud.
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Trump has yet to concede the election, and the delay — which has extended for over a week — is already keeping Biden from receiving high-level intelligence briefings and complicating his team's plans to move swiftly on the coronavirus.
Biden's team has said the transition is progressing despite the Trump administration's continued refusal to recognize a new president-elect, but it acknowledged that the longer the delay goes on, the worse its effects will be.
Biden's incoming White House chief of staff, Ron Klain, said on NBC News' "Meet the Press" that Trump's early morning tweet was "a further confirmation of the reality that Joe Biden won the election."
"And not through any of the rest of that tweet, not through fraud or anything else the president is baselessly alleging," Klain added, saying Biden "won through more votes."
"What we really want to see this week is for the General Services Administration to issue that ascertainment, so we can" move forward on national security and Covid-19 issues, he said.
Trump's refusal to accept the election results was discussed at length on the Sunday political talk shows. Speaking on "Meet the Press," Arkansas Gov. Asa Hutchinson, a Republican, said he expects "Joe Biden to be the next president of the United States."
"It was good, actually, to see President Trump tweet out that [Biden] won," Hutchinson said. "I think that's a start of an acknowledgment, and it is very important that Joe Biden have access to the intelligence briefings to make sure that he is prepared."
Hutchinson pointed to the Department of Homeland Security's recent assessment "that there's not any evidence of rampant fraud in the election that would undermine the result," he said, adding, "We need to accept the result."
Speaking on ABC News' "This Week," Trump's former national security adviser-turned-critic John Bolton said he believes "it's very important for leaders of the Republican Party to explain to our voters, who are not as stupid as the Democrats think, that in fact Trump has lost the election and his claims of election fraud are baseless."
"The fact is that we've seen litigation in all the key battleground states, and it has failed consistently," Bolton said. "Right now the Trump campaign is doing the legal equivalent of pitching pennies. Where are their silver dollars? Where is the evidence?"
Bolton said that as each day passes, it has become "clearer and clearer there isn't any evidence."
Rudy Giuliani, who is leading the president's legal efforts, said in an interview Sunday with Fox News that he has new evidence of electoral malfeasance that he cannot yet share. So far, Trump's legal efforts have fallen flat on their face, with his team losing over and over again in courts across a number of critical swing states.
In an interview with CBS News that aired Sunday, former President Barack Obama said there are "no factual scenarios" in which Trump wins the election.
"But there's damage to this, because what happens is that the peaceful transfer of power, the notion that any of us who attain an elected office — whether it's dogcatcher or president — are servants of the people," he said. "It's a temporary job."
"We're not above the rules," he added. "We're not above the law. That's the essence of our democracy."
Sen. Bernie Sanders, I-Vt., said on CNN's "State of the Union" that "the idea that he continues to tell his supporters that the only reason he may have lost this election is because of fraud is an absolutely disgraceful, un-American thing to do."
"Trump will have the distinction of doing more than any person in the history of this country in undermining American democracy," Sanders said.