It's the closest he has come to conceding the November 3 election, even as he repeats unfounded claims of massive voter fraud.
Speaking to reporters on the Thanksgiving holiday on Thursday, Trump said if Biden - who is due to be sworn in on January 20 - is certified the election winner by the Electoral College, he will depart.
But Trump said it would be hard for him to concede under the current circumstances and declined to say whether he would attend Biden's inauguration.
"This election was a fraud," he insisted in a sometimes rambling discourse at the White House, while continuing to offer no concrete evidence of widespread voting irregularities.
Biden won the election with 306 Electoral College votes - many more than the 270 required - to Trump's 232, and the electors are scheduled to meet on December 14 to formalise the outcome.
Biden also leads Trump by more than 6 million in the popular vote tally.
Trump has so far refused to fully acknowledge his defeat, though last week - with mounting pressure from his own Republican ranks - he agreed to let Biden's transition process officially proceed.
Asked if he would leave the White House if the Electoral College votes for Biden, Trump said: "Certainly I will. Certainly I will. And you know that."
"But I think that there will be a lot of things happening between now and the 20th of January. A lot of things," he said. "Massive fraud has been found. We're like a third world country."
Desperate efforts by Trump and his aides to overturn results in key states, either by lawsuits or pressuring state legislators, have failed, and he is running out of options.
In the United States, a candidate becomes president by securing the most "electoral" votes rather than winning a majority of the national popular vote.
Electors, allotted to the 50 states and the District of Columbia largely based on their population, are party loyalists who pledge to support the candidate who won the popular vote in their state.
Biden and Trump both stayed close to home to celebrate Thanksgiving on Thursday as the COVID-19 pandemic raged across the country.
Biden spent the holiday in the small seaside town of Rehoboth, Delaware, where he and wife Jill have a holiday home.
The former vice president, appearing with his wife in a video message posted to his Twitter account, said his family typically holds a large gathering on the island of Nantucket off Massachusetts but would remain in Delaware this year.
In the presidential-style address to a nation that has lost more than 260,000 lives to the coronavirus, the president-elect said Americans were making a "shared sacrifice for the whole country" and a "statement of common purpose" by staying at home with their immediate families.
"I know this isn't the way many of us hoped we'd spend our holiday. We know that a small act of staying home is a gift to our fellow Americans," Biden said.
"I know better days are coming."