Rising Springboks star Aphiwe Dyantyi, who won World Rugby's breakthrough player award last year, failed a doping test at a training camp with the South Africa squad last month.
Dyantyi said he was informed by South Africa's anti-doping agency that a urine sample he gave at the camp on July 2 tested positive for a banned substance.
He denied "ever taking any prohibited substance, intentionally or negligently, to enhance my performance on the field."
"I believe in hard work and fair play. I have never cheated and never will," Dyantyi said in a statement.
He requested his 'B' sample be tested. The substance Dyantyi tested positive for wasn't named. He said he was also tested two weeks before, on June 15, and that test was clear.
Dyantyi, who turns 25 next week, has scored six tries in 13 tests, including two in South Africa's 36-34 victory over the All Blacks in New Zealand last year.
The wing had a stellar start to his international career, leading to him being recognised as 2018's best newcomer by World Rugby, but he last played a test in November and has been limited by injury this season.
Dyantyi wasn't part of a Springboks training camp in Bloemfontein last week and wasn't expected to be included in South Africa's Rugby World Cup squad, which will be named on Monday.
He apologised to his teammates, family and friends for "the negative impact this news may have" and said he was focused on proving his innocence.