Trump had on a dark mask depicting the presidential seal as he walked outside Washington to greet wounded veterans through the halls of Walter Reed Military hospital.
Trump said to reporters as he left the White House: "If you're in a hospital, particularly ... I think wearing a mask is necessary."
The only other time Trump was seen wearing a mask during a May tour of a Ford factory, from which a photo appeared, was behind the scenes.
He refused to wear a mask in public at the time because he said he would not give the pleasure to the gathered press.
During the pandemic, the president was a latecomer wearing a mask that has spread across the World since March, infecting over 3.2 million people and killing at least 134,000 people. Most influential Republicans, including Vice President Mike Pence, also advocated wearing masks this summer as coronavirus gained ground. Republican governors have been moving towards allowing or promoting mask usage as the pandemic has become more extreme in some southern and western states.