Video showed Trump, wearing a black mask, waving from the back seat of a black four-wheel drive with two men – also masked – in the front seats.
"President Trump took a short, last-minute motorcade ride to wave to his supporters outside and has now returned to the Presidential Suite inside Walter Reed," White House spokesman Judd Deere said in a statement. "Appropriate measures" had been taken, he added.
But experts pointed out that the outing broke his own government's public health guidelines requiring patients to isolate while they are in treatment and still shedding virus – and endangered his Secret Service protection.
The US president has been criticised over his handling of the coronavirus crisis that has left nearly 210,000 Americans dead, the highest death toll anywhere in the world.
Trump, who has been repeatedly rebuked for flouting public health guidelines, said in a video that dropped on Twitter just before the appearance that he had "learned a lot about COVID" by "really going to school" as he has battled the virus.
"This is the real school. This isn't the "let's read the books school,' and I get it, and I understand it, and it's a very interesting thing," he added.
"This is insanity' Health experts criticised the "stunt," which they said demonstrated that he had learned nothing at all.
"Every single person in the vehicle during that completely unnecessary presidential "drive-by' just now has to be quarantined for 14 days," said James Phillips, chief of disaster medicine at George Washington University.
"They might get sick. They may die. For political theatre. Commanded by Trump to put their lives at risk for theatre. This is insanity," said Phillips, who is also an attending physician at Walter Reed military hospital near Washington, DC.
Trump was admitted to Walter Reed on Friday shortly after he was diagnosed with COVID-19. The 74-year-old's doctors have said he was given oxygen, as well as experimental treatments – which are usually used for people who have more severe cases of the disease – including the steroid dexamethasone.
The US president has posted videos and photos about his experience in the hospital on Twitter since he was admitted.
"We're going to pay a little surprise to some of the great patriots that we have out on the street," Trump said in his most recent video.
People with COVID-19 are generally required to quarantine for 14 days to avoid infecting others and the virus spreads more easily in indoor, confined environments.
"Beyond the ethical, clinical, epidemiological and health implications of this drive by, it shows a dire obsession with showing the public he's still in control, creating a false sense of normalcy and trying to normalize a highly transmissible virus," Dr Syra Madad, senior director of Special Pathogens at NYC Health + Hospitals, wrote on Twitter.
Senator Brian Schatz, a Hawaii Democrat, added that he was praying for the health of Trump's Secret Service security detail: "They understood the risk inherent in the job, but they shouldn't have to contemplate that the risk is coming from the protectee." The White House Correspondents Association issued a statement saying it was "outrageous" that Trump left the hospital without a press pool present. "The American public deserves independent coverage of the president so they can be reliably informed about his health."
here were, however, some messages of support. Texas Governor Greg Abbott, a Republican, reacted to news of the motorcade by tweeting: "Looks like ?@realDonaldTrump is making a swift recovery from COVID. So many prayers answered."
The episode came hours after a briefing by Trump's medical team, who said he had "continued to improve" and could return to the White House – which has the facilities to treat and isolate him – as early as Monday.
Trump has consistently downplayed the pandemic and, prior to his diagnosis, rarely wore a mask. The World Health Organization (WHO) and medical experts say face masks, hand-washing and physical distancing are key to curbing the spread of COVID-19.
Health experts have complained that the messaging from the administration – and particularly Trump's medical team – has caused widespread confusion.
Trump's physician Sean Conley admitted on Sunday that he had kept from the public the fact that the president had been given extra oxygen, in a bid to reflect an "upbeat attitude."
And he gave a rosy account of Trump's progress Saturday, only for White House chief of staff Mark Meadows to tell reporters immediately after that Trump's condition had been "very concerning" and that he was "still not on a clear path to a full recovery."
Trump and his wife Melania were confirmed to have the disease after his close adviser Hope Hicks was diagnosed with COVID-19 last week. A number of the president's close allies have also tested positive including his campaign manager Bill Stepien, former White House senior adviser Kellyanne Conway and former New Jersey Governor Chris Christie.
The election takes place in less than a month.