Mocanu, who turned 20 in May, was the surprise winner of the 100 and 200 metres backstroke at the 2000 Games in Sydney, becoming the first Romanian swimmer to win an Olympic gold medal.
She also won gold at the World Championships in the longer distance a year later but her form dipped thereafter and she failed to qualify for this year's Olympics in Athens.
"My decision is final," Mocanu told Reuters. "I totally lost my determination as an athlete."
"My future is now with coaching," said Mocanu, who is a student at a local physical education college.
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