The organization said in a statement that Johnson experienced symptoms common to the novel coronavirus, prompting him to alert officials and take a test. Players are tested before traveling each week and on site as part of the Tour's coronavirus testing protocols.
"Obviously, I am very disappointed," said Johnson, who won the Tour Championship to capture the FedEx Cup title last month. "I was really looking forward to competing this week but will do everything I can to return as quickly as possible.
"I have already had a few calls with the tour's medical team and appreciate all the support and guidance they have given me."
David Winkle, Johnson's agent at Hambric Sports, told ESPN that Johnson took an on-site COVID-19 test Sunday that came back negative. But the 23-time PGA Tour winner began feeling symptoms Sunday night and stayed away from the course Monday. He didn't feel better Tuesday, leading to the positive test result.
Since the symptoms started Sunday, it's possible Johnson could return at next week's Zozo Championship if his symptoms have subsided. He would be unable to practice on site before the tournament.
The 36-year-old Johnson hasn't played since he finished in a tie for sixth at the U.S. Open last month. J.T. Poston was the first alternate and replaces Johnson in the 78-player field.
The CJ Cup begins Thursday at Shadow Creek Golf Course in Las Vegas.