Thorpe has consistently denied any wrongdoing
Australia's anti-doping body has cleared five-times Olympic champion Ian Thorpe of using banned performance-enhancing drugs.
The 24-year-old swimmer returned a drug test in May 2006 with elevated levels of two hormones.
But experts concluded that the hormones were naturally-occurring and the Australian Sports Anti-Doping Authority said Thorpe had no case to answer.
Thorpe, who has retired from swimming, welcomed the decision.
"My reputation as a fair competitor in swimming is the thing I value most," he said in a statement.
"I have always been, and remain, a strong supporter of anti-doping testing."
Thorpe's case became public in March this year when a French newspaper published details of the test.
It said that his urine sample contained abnormal levels of testosterone and leutenising hormone.
The swimmer denied any wrongdoing and argued that the substances had occurred naturally in his body.
ASADA said it sought input from medical professionals and scientists around the world.
"Experts from these internationally respected organisations were unanimous in their opinion that the evidence available does not indicate the use of performance enhancing substances by the athlete," it said in a statement.
Thorpe won five Olympic and 11 World Championship titles before he retired in November 2006.