Tatum reportedly entered a quarantine period immediately that will last 10-14 days.
Boston has since shut down its practice facility temporarily, Washburn reports.
The 22-year-old forward has been listed as doubtful for the Celtics' game Sunday against the Miami Heat. Teammate Jaylen Brown has also been listed as questionable because of health and safety protocols.
Tatum is reportedly the second Celtics player to test positive for COVID-19 after center Robert Williams. Williams reportedly registered a positive result Friday, leading to mandatory seven-day quarantines for Tristan Thompson and Grant Williams.
Tatum is recording career-high averages across the board over 10 games this season. He's averaging 26.9 points, 7.1 rebounds, and 3.8 assists while shooting 43.8% on threes in his fourth campaign.