Caster Semenya won a gold medal in the 800 metres in Rio.
The 25-year-old streaked away from Francine Niyonsaba of Burundi and Kenya's Margaret Wambui, with Britain's Lynsey Sharp finishing sixth in a personal best of one minute 57.69 seconds at the Olympic Stadium.
Semenya has been mired in a gender row since winning the world title in 2009.
The Court of Arbitration for Sport last year suspended the rules of athletics' world governing body, the IAAF, on "hyperandrogenism", which causes unnaturally high levels of testosterone, meaning athletes on testosterone-suppressing medication, like Semenya, could come off it.
But fans were quick to show their support and congratulate the South African athlete:
Earlier this year, Semenya set a new personal best of 1min 55.33secs, the fastest time in the world since 2008, and she bettered that time in Rio by 0.05s.
Semenya's performance drew praise from Paula Radcliffe.
"That was really quick through 600 metres but Semenya had enough left to win the title convincingly," she told the BBC. "Semenya was so strong. I do think she's capable of breaking the world record. She definitely has it in her."