The men's 400 metres event at the 2016 Summer Olympics took place between 12 and 14 August at the Olympic Stadium. Fifty-three athletes from 35 nations competed. The event was won by 0.73 seconds by Wayde van Niekerk of South Africa, who broke the world record in the 400m and won the nation's second gold medal in the men's 400 metres. Kirani James of Grenada and LaShawn Merritt of the United States became the sixth and seventh men to win two medals in the event, but Michael Johnson remained the only man with two gold medals.

Wayde van Niekerk is a South African track and field sprinter who competes in the 200 and 400 metres. In the 400 metres, he is the current world and Olympic record holder, having set the record in the Olympic finals.