He was admitted at a London NHS hospital, believed to be St Thomas' in Westminster, in what his handlers said he was to undergo tests merely as a precaution, as reported by Daily Mail.
Johnson tested positive the last week in March.
Johnson's office said he was hospitalised because he still has symptoms 10 days after testing positive for the virus.
He said while Mr Johnson is not quite there yet, the Prime Minister has still been "heavily involved" in the government's efforts.
Johnson ordered a national lockdown on March 23 and has been shut away in his Downing Street apartment, in accordance with his own government's guidelines.
Health Secretary Matt Hancock returned to work on Friday after a week at home following his positive test for COVID-19.
Last night Downing Street insisted Mr Johnson had mild symptoms and would be able to continue with his duties.
"Although I'm feeling better and I've done my seven days of isolation, alas I still have one of the symptoms, a minor symptom, I still have a temperature", a weary-looking Johnson, sitting in a chair with his shirt open at the neck, said in a Twitter video message on Friday.
Johnson's fiancee Carrie Symonds, 32, revealed Saturday that she spent a week with coronavirus symptoms, though she wasn't tested.
She warned the country "may have more still to endure", but echoed the words of British armed forces' sweetheart Vera Lynne as she said: "We will meet again".
The Department of Health said there were now 47,806 positive cases, and 4,934 deaths.
News of the prime minister's hospitalization comes just hours after a rare televised address to the nation by Queen Elizabeth, rallying the United Kingdom against the coronavirus.