The US President had ended his self-isolation on Wednesday after what the White House described as a “mild” case of Covid.
However, on Saturday, Mr Biden announced he would be returning to self-isolation after testing positive once more, but said he had no symptoms and that he would continue working.
The 79-year-old said: “Folks, today I tested positive for COVID again. This happens with a small minority of folks.
“I’ve got no symptoms but I am going to isolate for the safety of everyone around me.
“I’m still at work, and will be back on the road soon.”
The White House doctor, Dr Kevin O’Connor, said the President tested negative on Tuesday evening, Wednesday morning, Thursday morning and Friday morning before testing positive on Saturday morning.
He said the infection appeared to be a mild but rare “rebound" case following treatment with an anti-viral drug.
Mr Biden had addressed the nation on Wednesday after appearing to get over his bout with the condition and ending isolation, saying he was “feeling great”.
He said that his condition had been mild because of a combination of vaccines and the treatment Paxlovid.
He said: “I got through it with no fear. A very mild discomfort because of these essentials, lifesaving tools.
“And guess what, I want to remind everybody: They are free. They are convenient, and they are safe, and they work."
Dr O’Connor had previously warned that there was a potential for a rebound infection because Mr Biden had been taking Paxlovid.
He said there had been a small percentage of patients who have been treated with Paxlovid who experienced a rebound in their infection after treatment.
However, despite that, Paxlovid has been proven to significantly reduce severe disease and death among those most vulnerable to Covid.
