World champion Steven Gardiner of Bahamas powered through on the home stretch to add Olympic gold to his world title in the 400 metres.
Gardiner clocked 43.85 seconds and silver went to Colombian Anthony Jose Zambrano in 44.08. Kirani James of Grenada completed his medal collection with bronze in 44.19 seconds, after 2012 gold and 2016 silver.
The Americans missed the podium with Michael Cherry fourth ahead of Michael Norman who led early but faded on the home stretch in the native country of his mother.
World record holder and 2016 winner Wayde van Niekerk of South Africa went out in the semi-finals.