With the disqualification of Nadzeya Ostapchuk, the gold medal was awarded to Valerie Adams of New Zealand — who winds up as Olympic champion for the second time in a row.
The International Olympic Committee said Ostapchuk, a former world champion, tested positive for steroids both before and after winning the shot put last week for her first Olympic gold.
After an IOC hearing, she was formally expelled from the games and had her victory and medal removed from the records. She was the eighth athlete, and first medalist, caught during the IOC's London drug-testing program.
"Catching cheats like this sends a message to all those who dope that we will catch them," IOC spokesman Mark Adams told The Associated Press.
Track and field's governing body, the IAAF, will consider further action against Ostapchuk, who could face a two-year ban from the sport.
Adams was bumped up from Olympic silver to gold, with Evgeniia Kolodko of Russia upgraded to silver and fourth-place finisher Gong Lijiao of China moving up to bronze.
Adams now has a second gold to go with her victory in Beijing four years ago.
"I am speechless with this news," she told New Zealand's national broadcaster TVNZ from her base in Switzerland. "It is taking me some time to take this in. It is huge and I am absolutely thrilled of course. It makes me extraordinarily proud as a New Zealander.
"It is also encouraging for those athletes, like myself who are proud to compete cleanly, that the system works and doping cheats are caught."
Adams' New Zealand teammates were ecstatic when they learned she would get the gold after all.
"Everyone cried — it was pretty emotional," said Annalie Longo, a member of the women's football team, as she left the athletes village Monday. "We're just so happy, she worked so hard and she totally deserved the gold medal."
She expressed regret that Adams had not been able to receive the gold medal at the victory ceremony.
"We feel cheated in a way, not being able to play the anthem and having taken the moment off her," Longo said.
The IOC took more than 5,000 urine and blood samples in London, including no-notice controls conducted outside competition. Until the shot put case, the games were set to end with medal standings in all 302 events unaltered by doping scandals.
The 31-year-old Ostapchuk, world champion in 2005, recorded the biggest shot put mark in a decade in the lead up to the Olympics. She won the gold with a mark of 21.36 meters.
The IOC said she tested positive for the steroid metenolone on Aug. 5, a day before her competition, and immediately after she won the event. The "A'' and the backup "B'' samples from both tests came back positive.
Ostapchuk told media in Belarus that she had done nothing wrong.
"I do not understand where it could come from," she told internet news agency Noviny.by. "I'm looking like an idiot to take this in heading for the games and knowing that it is so easy to be tested. Nonsense. I'm being tested every month, every week.
"I hope for the better. The most important for me is to clear my reputation. I've been in the sports for so many years and have never faced any claims. And now at the major event and after the gold medal .... I do not understand it."
A hearing was held Sunday, a few hours before the closing ceremony, attended by three Belarus team officials. They told the IOC that Ostapchuk had been tested in Belarus on July 25, July 26 and Aug. 1, and the results were negative. The athlete arrived in London on Aug. 4 and went straight to the athletes village.
"They had no explanation as to why such a substance would have been found in the sample of the athlete," the IOC said.
After seeing the test results, the Belarus team did not contest that the steroids were found in her system.
The Belarus Olympic Committee and national anti-doping agency will investigate and "take the appropriate measures," the IOC said.
IAAF scientific expert Jordi Segura said metenolone could be taken either orally or by injection, and the substance can be eliminated from the system within four to five days, the IOC said.
It's the second Olympic doping scandal here for Belarus. The team had earlier sent home hammer thrower Ivan Tsikhan after retests of his samples from the 2004 Athens Olympics, where he won silver, came back positive.
Besides Ostapchuk, only one athlete tested positive for a banned substance after competing in London. U.S. judo fighter Nick Delpopolo was cited for traces of marijuana in his urine sample.
"This is the eighth athlete to have been caught by our extensive in-games testing but many more were kept away from the games by the pre-games program," Adams said. "And keeping samples for eight years means that we can continue to test long after the competition is over."
Three Beijing events were tainted by drug scandals during the 2008 Games, and two more medals were changed months later when a new test for the blood-booster CERA was introduced. The signature men's 1,500-meter gold medal was stripped retroactively from Rashid Ramzi of Bahrain.
