South Africa’s Caster Semenya won Olympic gold in the 800m but Great Britain’s Lynsey Sharp missed out on a medal as she finished sixth.
Semenya set a new national record to win in one minute 55.28 seconds and finish well clear of silver medallist Francine Niyonsaba of Burundi.
Kenya’s Margaret Nyairera Wambui took the bronze medal, while Scot Sharp set a new personal best of 1:57:69.
Semenya, 25, has now won two Olympic medals, after a silver at London 2012.
There she finished behind Russia’s Mariya Savinova, who has since been named in a doping report, with the World Anti-Doping Agency saying she should receive a lifetime ban.
Four years on, Semenya finished more than a second clear of Niyonsaba to take the title.
Her time was two seconds adrift of the world record of 1:53:28, set by Jarmila Kratochvilova, running for Czechoslovakia in July 1983.
Semenya has faced continued questions over her eligibility to race after she was subject to gender testing after winning the world title in 2009.
She has since been diagnosed with hyperandrogenism, which means her testosterone levels are far in excess of the vast majority of women.
Semenya was cleared to compete by the International Association of Athletics Federations in 2010 after being sidelined for 11 months while she had tests.
New regulations requiring female athletes to take testosterone-lowering medication if their natural levels were above the legal mark were suspended for two years by the Court of Arbitration for Sport last July.
Former marathon world champion Paula Radcliffe says:
“That was really quick through 600 metres but Semenya had enough left to win the title convincingly.
“Semenya was so strong. I do think she’s capable of breaking the world record. She definitely has it in her.”