The US president donned a mask during a visit to a military hospital on Saturday.
Setting out from the White House, he told reporters: “When you’re in a hospital, especially… I think it’s expected to wear a mask.”
Mr Trump flew by helicopter to Walter Reed National Military Medical Centre in suburban Washington to meet wounded service-members and health workers caring for Covid-19 patients.
The president was pictured wearing a black mouth and nose covering as he began his tour of the facility.
The move is significant owing to Mr Trump’s longstanding refusal to wear a mask at news conferences, coronavirus task force updates, rallies and other public events.
Sources close to him have suggested he feared that a mask would make him look weak.
While not wearing one himself, President Trump has sent mixed signals about masks, acknowledging that they would be appropriate if worn in an indoor setting where people were close together.
At the same time, he has accused reporters of sporting them to be “politically correct”, and has also retweeted messages deriding his Democratic rival Joe Biden for wearing one.
The wearing of masks has become a political dividing line in the country, with Republicans more resistant to wearing them than Democrats, according to nationwide polling.
Few masks were seen at recent Trump rallies in Tulsa, Oklahoma, Phoenix and South Dakota’s Mount Rushmore.
The only time Mr Trump has been known to wear a mask was during a private part of a tour of a Ford plant in Michigan.