Biden, 81, in a post on X, said he will remain in his role as president and commander-in-chief until his term ends in January 2025 and will address the nation this week.
"It has been the greatest honor of my life to serve as your President.
"And while it has been my intention to seek reelection, I believe it is in the best interest of my party and the country for me to stand down and to focus solely on fulfilling my duties as President for the remainder of my term," Biden wrote.
His initial statement had not included an endorsement of Harris, but he followed up a few minutes later with an expression of support.
Harris, 59, would become the first Black woman to do run at the top of a major-party ticket in the country's history.
It was unclear whether other senior Democrats would challenge Harris for the party's nomination - she was widely seen as the pick for many party officials - or whether the party itself would choose to open the field for nominations.
Biden's announcement follows a wave of public and private pressure from Democratic lawmakers and party officials to quit the race after his shockingly poor performance in a June 27 televised debate last month against Republican rival Trump, 78.
Biden's failure at times to complete clear sentences took the public spotlight away from Trump's performance, in which he made a string of false statements, and trained it instead on questions surrounding Biden's fitness for another 4-year term.
Days later he raised fresh concerns in an interview, shrugging off Democrats' worries and a widening gap in opinion polls, and saying he would be fine losing to Trump if he knew he'd "gave it my all."
His gaffes at a NATO summit - invoking Russian President Vladimir Putin's name when he meant Ukrainian President Volodymyr Zelenskiy and calling Harris "Vice President Trump" -further stoked anxieties.
Only four days before Sunday's announcement, Biden was diagnosed with COVID-19 for a third time, forcing him to cut short a campaign trip to Las Vegas. More than one in 10 congressional Democrats had called publicly for him to quit the race.
Biden's historic move - the first sitting president to give up his party's nomination for reelection since President Lyndon Johnson in March 1968 - leaves his replacement with less than four months to wage a campaign.
If Harris emerges as the nominee, the move would represent an unprecedented gamble by the Democratic Party: its first Black and Asian American woman to run for the White House in a country that has elected one Black president and never a woman president in more than two centuries of democracy.
Biden was the oldest US president ever elected when he beat Trump in 2020. During that campaign, Biden described himself as a bridge to the next generation of Democratic leaders. Some interpreted that to mean he would serve one term, a transitional figure who beat Trump and brought his party back to power.
But he set his sights on a second term in the belief that he was the only Democrat who could beat Trump again amid questions about Harris's experience and popularity. In recent times, though, his advanced age began to show through more. His gait became stilted and his childhood stutter occasionally returned.
His team had hoped a strong performance at the June 27 debate would ease concerns over his age. It did the opposite: a Reuters/Ipsos poll after the debate showed that about 40% of Democrats thought he should quit the race.
Donors began to revolt and supporters of Harris began to coalesce around her. Top Democrats, including former House of Representatives Speaker Nancy Pelosi, a longtime ally, told Biden he cannot win the election.
Biden initially resisted pressure to step aside. He held damage-control calls and meetings with lawmakers and state governors, and sat for rare television interviews. But it was not enough. Polls showed Trump's lead in key battleground states widening, and Democrats began to fear a wipeout in the House and Senate. On July 17, California's Rep. Adam Schiff called on him to exit the race.
Biden's departure sets up a stark new contrast, between the Democrats' presumptive new nominee, Harris, a former prosecutor, and Trump who at 78 is two decades her senior and faces two outstanding criminal prosecutions related to his attempts to overturn the 2020 election result. He is due to be sentenced in New York in September on a conviction for trying to cover up a hush-money payment to a porn star.
BIDEN STRUGGLED BEFORE DEBATE
Earlier this year, facing little opposition, Biden easily won the Democratic Party's primary race to pick its presidential candidate, despite voter concerns about his age.
However, his staunch support for Israel's military campaign in Gaza eroded support among some in his own party, particularly young, progressive Democrats and voters of color.
Many Black voters say Biden has not done enough for them, and enthusiasm among Democrats overall for a second Biden term had been low. Even before the debate with Trump, Biden was trailing the Republican in some national polls and in the battleground states he would have needed to win to prevail on Nov. 5.
Harris was tasked with reaching out to those voters in recent months.
During the primary race, Biden accumulated more than 3,600 delegates to the Democratic National Convention to be held in Chicago in August. That was almost double the 1,976 needed to win the party's nomination.
Unless the Democratic Party changes the rules, delegates pledged to Biden would enter the convention “uncommitted," leaving them to vote on his successor.
Democrats also have a system of “superdelegates," unpledged senior party officials and elected leaders whose support is limited on the first ballot but who could play a decisive role in subsequent rounds.
Biden beat Trump in 2020 by winning in the key battleground states, including tight races in Pennsylvania and Georgia. At a national level, he bested Trump by more than 7 million votes, capturing 51.3% of the popular vote to Trump's 46.8%.
