Michael Phelps became the first swimmer to win four successive Olympic gold medals in one event, when he won the 200m individual medley.
It was the 22nd Olympic gold medal for the 31-year-old Phelps, a world record.
In gymnastics, American Simone Biles won the women's all-around Olympic gold medal. The 19-year-old had already led the US to victory in the team event.