Jamaica's Veronica Campbell held off a fierce challenge from 18-year-old Allyson Felix to take 200m gold.
Campbell, the pre-race favourite, led with 80m to go and kept her form in the last few strides to take her country's first 200m Olympic title.
Felix took silver in 22.18secs, a new world junior record, with Debbie Ferguson of the Bahamas in bronze.
Britain's Abi Oyepitan finished joint seventh in her first major final after running personal bests in qualifying.
Campbell, the bronze medallist from the 100m, ran 22.05 seconds, the fastest time in
the event since 1999.
She became only the second Jamaican woman track athlete
to win Olympic gold - Deon Hemmings won the 400m hurdles in
"I am really delighted, my training has been going great and I knew I was capable of this," said Campbell.
"I have been dreaming of a gold medal for a long time. I felt Felix coming up to me 20-30 metres mark but she ran out of room."
Oyepitan said: "I was so tired. I didn't realise until I started running. I thought I could work the straight but I had nothing left.
"But I've done several PBs and I've got into the Olympic final, so I'm pleased."