As of July 21, the US has recorded 3,825,153 COVID-19 cases. It has also recorded 140,957 deaths, according to Johns Hopkins University. Mr Trump has previously dismissed the need for face masks despite the Centres for Disease Control and Prevention recommending their use over three months ago.
Mr Trump tweeted on Monday that wearing a face mask when social distancing is impossible is “patriotic”.
He said: “We are United in our effort to defeat the Invisible China Virus, and many people say that it is Patriotic to wear a face mask when you can’t socially distance.
“There is nobody more Patriotic than me, your favourite President!”
It is the president’s strongest endorsement of masks to date.
Mr Trump’s tweet follows him refusing to issue a national mandate requiring people to wear face masks.
When asked by Fox News whether he would consider making masks mandatory, Mr Trump replied that he wanted “people to have a certain freedom” in whether they wear one.
He added: “I don’t agree with the statement that if everybody wears a mask, everything disappears, masks cause problems, too.
“With that being said, I’m a believer in masks. I think masks are good.
“But I leave it up to the governors.”
Mr Trump was first spotted wearing a face mask when he visited Walter Reed National Military Medical Center in Maryland this month.
He said to Fox Business after the trip that he changed his mind on masks as he “sort of like the way I looked”.
He added: “It looked like the Lone Ranger, I have no problem with that, and if people feel good about it, they should do it.”
Mr Trump also visited a medical supply company in Maine that created COVID-19 tests in June, without wearing a mask.
Puritan Medical Products announced after his visit that products made while Mr Trump was present would be destroyed to prevent cross-contamination.
Wearing face masks has become a political issue in the US, with states divided on whether to make their usage mandatory.
In May, Mr Trump shared a tweet mocking Democrat rival Joe Biden for wearing a face mask to a Memorial Day ceremony.
Georgia Governor Brian Kemp also sued the Mayor of Atlanta to prevent her from mandating masks in the city.
Atlanta Mayor Keisha Lance Bottoms savaged Mr Kemp for the legal action, and said: “Brian Kemp does the bidding of President Trump.”
Mr Kemp has urged all Georgia residents to wear face masks for at least four weeks, but has stopped short of mandating them due to fears masks would cripple the economy.