Matt Centrowitz of the United States ran from the front and held off Taoufik Makhloufi of Algeria at the line to win the gold medal in the men's 1,500 meters.
Centrowitz won the silver medal at the 2013 world championships and took fourth at the 2012 London Games. His last lap on Saturday was 50.62 seconds.
Nick Willis of New Zealand took bronze.
The last American to win the men's 1,500 was Mel Sheppard at the 1908 London Olympics.
Kenya favorite Asbel Kiprop fell halfway through the race and could not get back into medal contention. He finished sixth.
Mo Farah of Britain became the first runner in 40 years to win back-to-back long-distance doubles at the Olympics by taking gold in the men's 5,000 meters.
Farah already won the 10,000 meters at the Rio de Janeiro Games to go along with his two gold medals from the same events in London four years ago.
The last man to win both distance races in consecutive Olympics was Finnish great Lasse Viren at the 1972 Munich and 1976 Montreal Games.
Farah won 13 minutes 3.30 seconds. American Paul Chemilo took second and Hagos Gebrhiwet of Ethiopia was third. Chemilo and fourth-lacer Mohammed Ahmed of Canada were originally disqualified but won an appeal and were reinstated. That took the bronze away from Bernard Lagat of the United States, who originally finished fifth.
Caster Semenya of South Africa won the women's 800 meters gold medal, fighting from behind to overtake Francine Niyonsaba of Burundi on the final bend before cruising home.
Semenya won in a South African national record 1:55.28 seconds, while Niyoinsaba of Burundi was 1.21 seconds behind. Margaret Wambui of Kenya took bronze in 1:56.89.
