"Don tested positive at the start of the week and has been quarantining out at his cabin since the result," his spokesman said.
"He's been completely asymptomatic so far and is following all medically recommended Covid-19 guidelines," the spokesman said of the 42-year-old.
Trump Jr's positive test follows infections for a list of others connected to the White House, including his father, the president's wife Melania and Trump's youngest son Barron.
Earlier Friday, Andrew Giuliani, a White House aide and son of the president's personal lawyer Rudy Giuliani, announced he had also tested positive.
Trump's administration has been heavily criticized for its response to the pandemic, while the president himself has repeatedly downplayed the danger of the virus.