Shelly-Ann Fraser-Pryce of Jamaica saw off the challenge of American Carmelita Jeter to retain her Olympic 100m title in London.
Fraser-Pryce moved ahead at the halfway stage and held on in a close finish to win in 10.75 seconds.
Jeter beat another Jamaican, Veronica Campbell-Brown, on the line for bronze.
Fraser-Pryce becomes the first woman since American Gail Devers in 1992 and 1996 to win back-to-back 100m titles at Olympic Games.
"It's completely different to Beijing because there I was inexperienced," said the 25-year-old Jamiacan.
"I was young and I never believed I could win. This time I was a bit nervous but I believed in God and I trusted him to carry me through."