The 79-year-old, who is fully-vaccinated and has twice received booster jabs, is experiencing “very mild symptoms” and will continue to carry out all his duties, press secretary Karine Jean-Pierre said.
The president is taking the antiviral medicine Paxlovid, her statement added.
First Lady Jill Biden is said to have tested negative while in Delaware.
Consistent with White House protocol the president will continue to work in isolation until he tests negative, Ms Jean-Pierre said,
He will take part in meetings via telephone and Zoom, the official said.
White House COVID coordinator Dr Ashish Jha said the president was tired, with a runny nose and dry cough.
He said Mr Biden went to bed feeling fine but did not sleep well and then tested positive on Thursday morning.
Ms Jean-Pierre will hold a press briefing alongside Dr Jha later.
The White House medical unit will inform all close contacts of the president during the day, Ms Jean-Pierre said, including any members of Congress or members of the press who were with Mr Biden during his visit to Massachusetts on Wednesday.
He last tested negative for COVID on Tuesday, the White House said.
Paxlovid is an antiviral medicine which stops the COVID virus from multiplying in the body, keeping virus levels low in the body while the immune system fights the infection.
It is actually a course of two treatments or tablets. The patient usually takes the pills daily – one tablet of ritonavir and two of nirmatrelvir – for five days.
