Biden, 79, has a runny nose, fatigue and an occasional dry cough, symptoms which he began to experience late on Wednesday, White House physician Kevin O'Connor said in a note released on Thursday.
Biden has begun taking the antiviral treatment Paxlovid, O'Connor said.
"He is fully vaccinated and twice boosted and experiencing very mild symptoms," press secretary Karine Jean-Pierre said in a statement.
The White House will provide a daily update on Biden's health, and he will follow Centers for Disease Control and Prevention guidelines and isolate at the White House while continuing to work, she said.
Biden faces dual challenges of soaring inflation and Russia's land assault on Ukraine. His popularity has dropped sharply over the past year. A Reuters/Ipsos survey completed on Tuesday showed 36 percent of Americans approve of his job performance.
His illness forced cancellation of a trip to Pennsylvania where Biden intended to lay out plans to ask Congress for US$37 billion for crime prevention programs.
"Folks, I'm doing great. Thanks for your concern. Just called Senator Casey, Congressman Cartwright, and Mayor Cognetti (and my Scranton cousins!) to send my regrets for missing our event today. Keeping busy!" Biden wrote on Twitter.
A picture accompanying the tweet showed the president smiling, wearing a blazer and sitting at a desk with papers.
The White House provided an unusually detailed account of the president's morning activities, including a series of phone calls to political allies.
Harris, Pelosi test negative
Multiple members of Biden's administration and other senior figures in Washington have tested positive for the coronavirus in recent months, including Vice President Kamala Harris and House of Representatives Speaker Nancy Pelosi, both of whom have since tested negative and resumed working.
Representative Bennie Thompson, the chairman of the congressional probe of the 6 January assault on the US Capitol, plans to run Thursday night's hearing remotely as a result of testing positive.
US stocks briefly headed lower following reports of the president's diagnosis, with the S&P 500 dropping about 0.5 percent over the following 10 minutes. The index quickly retraced that loss and by mid-morning was back to near the unchanged mark on the day.
While many Americans have moved on from the strict precautions of the pandemic's early months, returning to offices and schools and resuming summer travel, the virus continues to spread.
Cases in the United States are up more than 25 percent in the last month, according to CDC data, as the rapidly spreading BA.5 subvariant has taken hold.
Evading the immune protection afforded either by vaccination or prior infection, BA.5 has been the dominant subvariant in the United States since at least early July and has driven a surge of new infections globally.
The Pfizer Inc antiviral drug Paxlovid that Biden is taking has been shown to reduce the risk of severe disease by nearly 90 percent in high risk patients if given within the first five days of infection.
But Paxlovid has in some cases been associated with rebound infections, in which patients improve quickly and test negative after a five-day course of the drug, with symptoms returning days later.
Dr. Bruce Farber, chief of infectious diseases at Northwell Health in New York, who is not treating the president, said Paxlovid is likely the only treatment Biden will get, unless his symptoms worsen.
"Elderly people are more at risk for developing complications from Covid," Farber said. "It dramatically is lower if you've been vaccinated and doubly boosted, which he has been, so I anticipate he will do very well."
'Get vaccinated now'
Biden set up strict Covid-19 safety protocols at the White House, urged Americans to take the virus seriously and campaigned for everyone to get fully vaccinated.
He is tested regularly for the disease and anyone who meets with him or travels with him is tested beforehand, the White House has said. Biden had last tested negative on Tuesday.
He has stopped wearing a mask at public events in recent months, and the White House dropped its mask requirement ahead of his 1 March State of the Union Address.
Asked by Reuters on Wednesday what the country should do with Covid cases on the rise, Biden encouraged vaccination.
"It's not in their interest or the public's interest not to get vaccinated," Biden, told reporters at Joint Base Andrews, referring to people who were not vaccinated. "We have the capacity to control it. They should get vaccinated now."
Biden joins a roster of other world leaders who have contracted Covid-19 since the pandemic started in early 2020.
A month before he lost the 2020 presidential election to Biden, his Republican predecessor Donald Trump was hit hard by the virus. He, his wife Melania and other White House staff contracted it after an event for Supreme Court Justice Amy Coney Barrett in September 2020.
Trump, then 74, was hospitalized on 2 October 2020, and underwent aggressive treatment at Walter Reed National Military Medical Center in a suburb of Washington. His low oxygen levels alarmed his medical team.
"We are in a very different place than the last time a POTUS got Covid," Ezekiel "Zeke" Emanuel, a physician and former member of Biden's Covid transition team, wrote on Twitter.
"Biden is fully vaxxed/boosted and reportedly on a therapeutic (Paxlovid) that significantly mitigates symptoms. If anything, this shows how contagious BA5 is. Feel better, @POTUS!"
Photo AFP Caption: US President Joe Biden