A female United States gymnast tested positive for Covid and a team member has been identified as a close contact.
The gymnast, who is not displaying any symptoms, tested positive at the team's training camp in the city of Inzai.
Both athletes have now been transferred to a hotel to quarantine with the rest of the squad having moved to athletes' village in Tokyo.
The US Olympic Committee (USOC) told BBC Sport the athlete was an alternate - a team member included as a reserve.
"The health and safety of our athletes, coaches and staff is our top priority," read a USOC statement.
"We can confirm that an alternate on the women's artistic gymnastics team tested positive for Covid-19.
"Out of respect for the individual's privacy, we cannot provide more information at this time."
Superstar gymnast Simone Biles, 24, is part of the US team competing at the Tokyo Games.
Biles needs just four more medals to surpass Vitaly Scherbo's all-time record of 33 Olympic and world medals, while the US have won team gymnastics gold at the last two Olympic Games.
On Monday afternoon Biles posted pictures on social media from inside the Olympic Village.
