The announcement that Nadzeya Ostapchuk had failed her tests came after International Olympic Committee president Jacques Rogge proclaimed the fight against doping at London 2012 a success.
Before Ostapchuk, only one athlete tested positive for a banned substance after competing. The US judoka Nick Delpopolo was cited for traces of marijuana in his urine sample.
He blamed "inadvertent consumption" of food baked with the substance. The IOC disqualified him from seventh place in the 73kg class.
Seven more were caught in doping controls conducted since the official testing period for the Games began on 16 July. One of the seven competed in London before her test result was known.
"I think that is a sign that the system works," Rogge said. "I am happy about the fact that we could catch athletes who cheated, both before the Games and at the Games."