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 public this week.
He has not been seen in public since testing positive for COVID-19 last week and isolating at his home in Rehoboth Beach, Delaware
"While it has been my intention to seek re-election, 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.
Democratic National Committee chair Jaime Harrison said the public will hear from the party on next steps and the path forward for the nomination process soon.
My fellow Democrats, I have decided not to accept the nomination and to focus all my energies on my duties as President for the remainder of my term. My very first decision as the party nominee in 2020 was to pick Kamala Harris as my Vice President. And it's been the best… — Joe Biden (@JoeBiden) pic.twitter.com/x8DnvuImJVJuly 21, 2024
It was the first time in more than a half-century that an incumbent US president gave up his party's nomination.
"My intention is to earn and win this nomination," Harris said in a statement.
"I will do everything in my power to unite the Democratic Party - and unite our nation - to defeat Donald Trump."
Biden's campaign had been on the ropes since a halting June 27 debate against Trump, 78, in which the incumbent at times struggled to finish his thoughts.
Opposition from within his party gained steam over the past week with 36 congressional Democrats - more than one in eight - publicly calling on him to end his campaign.
Lawmakers said they feared he could cost them not only the White House but also the chance to control either chamber of Congress next year, which would leave Democrats with no meaningful grasp on power in Washington DC.
That stood in sharp contrast to what played out in the Republican Party last week, when members united around Trump and his running mate US Senator JD Vance, 39.
Biden's move could clear the way for Harris, 59, to run at the top of the ticket.
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.
Trump, the Republican candidate in the November 5 election, told CNN on Sunday that he believed Harris would be easier to defeat.
— Joe Biden (@JoeBiden) pic.twitter.com/RMIRvlSOYwJuly 21, 2024
Congressional Republicans argued that Biden should resign the office immediately, which would turn the White House over to Harris and put House of Representatives Speaker Mike Johnson, a Republican, next in line in succession.
"If he's incapable of running for president, how is he capable of governing right now? I mean, there is five months left in this administration. It's a real concern, and it's a danger to the country," Johnson told CNN on Sunday before Biden's announcement.
Johnson in a separate interview on US broadcaster ABC signalled that Republicans would likely try to mount legal challenges to Democrats' move to replace Biden on the ballot.
Biden's announcement follows a wave of public and private pressure from Democratic lawmakers and party officials to quit the race after his poor debate.
Biden had a change of heart on Sunday, a source familiar with the matter told Reuters.
The president told allies that as of Saturday night he planned to stay in the race before changing his mind on Sunday afternoon.
"Last night the message was proceed with everything, full speed ahead," a source familiar with the matter told Reuters, speaking on condition of anonymity.
"At around 1.45pm today: the president told his senior team that he had changed his mind."
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.
Biden's historic move - the first sitting president to give up his party's nomination for re-election since President Lyndon Johnson in March 1968 - leaves his replacement with less than four months to wage a campaign.
Biden was the oldest US president ever elected when he beat Trump in 2020.
