Mr Trump had previously refused to wear masks in public, although he had said he would if in a crowd and could not maintain physical distance from others.
He had come under sharp criticisms even from his fellow Republican Party members, who said his stance showed lack of example or leadership.
He finally did that during a visit to the Walter Reed National Military Medical Centre outside Washington where he met wounded soldiers and frontline healthcare workers.
However, Mr Trump did not mention the coronavirus as a reason for wearing the mask.
“I’ve never been against masks but I do believe they have a time and a place,” he said earlier before leaving the White House.
Later, he said wearing a mask was the right thing to do “when you’re in a hospital … where you’re talking to a lot of soldiers and people that, in some cases, just got off the operating tables.
“I think it’s a great thing to wear a mask,” he declared.
Public health officials and institutions have been urging people to use masks to slow the spread of the virus.
As of Friday, the disease had claimed no fewer than 134,000 lives in the U.S., which has recorded no fewer than three million infections.
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