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."
The IOC had said this would be its most extensive Olympic anti-doping programme. Its checks included no-notice tests ahead of athletes competing.
Rogge cautioned that some samples were still being analysed and "we might hear something tomorrow or the day after. Hopefully not, but you never know."
The Syria runner Ghfran Almouhamad tested positive for the stimulant methylhexaneamine two days before her 400m hurdles heat. She was eighth, but was eliminated before the IOC disqualified her.
Until Ostapchuk's disgrace, London 2012 was poised to end with medal standings in all 302 events unaltered by doping scandals.
Three Beijing events were tainted during the Games, and two more medals were changed months later when a new test for the blood-booster Cera was introduced.
The signature men's 1500m gold medal was stripped from Rashid Ramzi of Bahrain.
Rogge said that the IOC would store all samples from London and could reanalyse them, revise results and reallocate medals until the statute of limitations expired in August 2020.