Quincy Hall came from behind to win the men's 400m final and claim his first Olympic gold medal.
It is the first U.S. gold in the event since LaShawn Merritt in 2004: the U.S. swept the medals that year.
It is the first medal for the United States since Merritt won bronze in 2016.
Hall finished the race in 43.40 seconds, a personal best (0.37 seconds shy of the world record).
It is the fourth individual gold medal for the United States in track and field, joining Noah Lyles (100m), Cole Hocker (1500m) and Ryan Crouser (shot put).
The 26-year-old finished third at the world championships last year but was part of the 4x400m relay team that won the gold medal at the event.