symptoms of Covid-19 after he and his wife Melania tested positive for the coronavirus, the White House says.
Mr Trump’s chief of staff said the president was “on the job” and “in good spirits”, adding that he expected him to make a quick recovery.
The news comes just over a month before presidential elections, where he will face Democratic challenger Joe Biden.
“I hope this serves as a reminder,” he tweeted after the result. “Wear a mask, keep social distance, and wash your hands.”
Other close members of Mr Trump’s family have tested negative.
Officials said the process of tracking all the president’s contacts in recent days was ongoing, adding that Mr Trump was considering how he might address the nation or otherwise communicate with the American people on Friday.
On Thursday, the first couple said they intended to self-isolate after one of Mr Trump’s closest aides, Hope Hicks, tested positive. Soon afterwards, they too received positive test results.
But there has been criticism of Mr Trump’s decision to go to a fundraiser attended by dozens of people in New Jersey on Thursday, apparently when officials already knew about Ms Hicks’s symptoms.
Ms Hicks, 31, travelled with Mr Trump on Air Force One to the first presidential TV debate with Mr Biden in Ohio on Tuesday. Some of Mr Trump’s family members who attended the debate were seen not wearing masks.
Mr Trump has mostly spurned mask-wearing and has often been pictured not socially distanced with aides or others during official engagements.
The coronavirus has infected more than 7.2 million Americans, killing more than 200,000 of them.
White House Chief of Staff Mark Meadows said Mr Trump was experiencing mild symptoms, but that he and Melania were “in good spirits”. He said he was optimistic the president would make a speedy recovery.
He was on the job and would remain on the job, Mr Meadows added, saying updates on his health were expected later on Friday.
Earlier Mr Trump wrote on Twitter: “We will get through this together.”
Melania Trump tweeted that she was looking forward to a speedy recovery.
“Thank you for the love you are sending our way. I have mild symptoms but overall feeling good,” she said.
Trump’s physician, Dr Sean Conley, released a statement late on Thursday, saying the president and the first lady were “both well at this time, and they plan to remain at home within the White House during their convalescence”.