Trump's tweet refers to Covid-19 as the "Invisible China Virus" – a name for the disease he has frequently been criticized for using – and features a black-and-white photo of him wearing a mask.
"Many people say that it is Patriotic to wear a face mask when you can't socially distance," Trump tweeted. "There is nobody more Patriotic than me, your favorite President!"
The tweet from Trump, appearing to endorse the use of face coverings in public, follows months of mixed messages on whether he believed they were an essential tool to help prevent transmission of the disease.
As multiple states experience a record-breaking surge in coronavirus cases, masks have been embraced by business leaders, politicians of both major parties and even Trump administration health officials.
But up until his tweet Monday, Trump has taken a less full-throated approach.
In a Fox News interview that aired Sunday, for instance, Trump said, "I don't agree with the statement that if everybody wear a mask everything disappears." The president was responding to remarks made by Dr. Robert Redfield, director of the Centers for Disease Control and Prevention and a member of the White House coronavirus task force, who said that if "everybody" wore masks, the virus could be under control in a matter of weeks.
"All of sudden everybody's got to wear a mask, and as you know masks cause problems, too," Trump told Fox.
But he added: "With that being said, I'm a believer in masks. I think masks are good."
Trump has largely refused to wear a mask in public. He notes that it's not necessary for him as president, because nearly everyone in his proximity is tested for Covid-19 before they approach him. But he has declined to don a face covering in numerous public settings throughout the crisis, including at a Honeywell factory in Arizona that produces masks.
When the CDC in early April changed its guidance to recommend using a face covering in public areas where social distancing was unfeasible, Trump made the announcement himself – but while delivering the news, he said, "I don't think I'm going to be doing it."
Trump's skepticism toward masks, as well as his broad refusal to wear one even as a symbolic gesture, have played a role in the issue dividing Americans along partisan lines, experts say.
"Unfortunately, as with much of science and health, the issue became extraordinarily politicized [during the pandemic] and I think the president bears a lot of responsibility for that," said Michael Sparer, professor and chair of health policy and management at Columbia University, in an interview with CNBC on Monday before Trump's tweet.
Only twice since the pandemic began has Trump been seen wearing a mask, and only once did he wear one in full view of the press.
He openly wore a mask for the first time just this month, during a visit to Walter Reed National Military Medical Center. Before arriving there, Trump told reporters, "I've never been against masks but I do believe they have a time and a place."
But it remained unclear at what time and place Trump considered masks appropriate.
"I think his messaging has been extraordinarily inconsistent," Sparer said. The implicit message of Trump wearing a mask in a hospital and taking it off in other public settings, the professor said, is that deciding when and where to wear a mask is an individual choice.
"We don't tell people they have the freedom of choice to wear a seat belt. We don't tell people they have the freedom of choice not to wear a motorcycle helmet," Sparer said. "There are certain public health actions that we take on behalf of the public."
The Trump administration has been criticized for failing to show strong leadership by allowing states to disregard the federal government's social distancing recommendations and set their own rules on how to combat the pandemic.
The administration has also repeatedly rebuffed pressure from Democrats and other groups to enact a national mask mandate.
Masks were not always championed by public health officials. In March and April, when hospitals in hot-spot states faced dire shortages of personal protective equipment, experts pushed back on Trump's own suggestion that Americans use scarves to cover their faces in public.
At that time, there was little empirical evidence to support the claim that cloth coverings could help slow the spread of the virus. But new information about asymptomatic transmission prompted the CDC and others to update their guidance shortly after.
As of Monday, more than half of all U.S. states have put in place statewide mandates requiring people to wear masks.
More than 140,000 people have died due to the coronavirus in the United States, according to a Johns Hopkins University tally. More than 3.8 million have been infected.
Trump's tweet also came after his announcement that he would resume daily coronavirus briefings at the White House, as his approval rating for handling the crisis has plummeted, and he has lost significant ground in polls to Democratic rival Joe Biden. The next briefing will be held Tuesday around 5 p.m. ET, Trump and White House press secretary Kayleigh McEnany said.