Uzbek gymnast Luiza Galiulina has been suspended after testing positive for diuretic furosemide, the International Olympic Committee (IOC) announced on Sunday.
Galiulina was tested July 25, four days before the start of the women's gymnastics competition, reports Chinese news agency Xinhua.
"Once the results of the B sample have been received, a definitive decision will be announced by the disciplinary commission," said the IOC.
Luiza Galiulina, 20, was found to have taken the diuretic that can mask the presence of other prohibited drugs in a sample submitted Wednesday, the IOC said.
It is the second doping-related exclusion during the Games after Albanian weightlifter Hysen Pulaku was banned Saturday after testing positive for steroids.
More than 100 athletes were prohibited from competing in London over doping offences that were uncovered in the six months leading up to the Games.