The men's 400 metres event at the 2020 Summer Olympics took place between 1 and 5 August 2021 at the Olympic Stadium. Approximately fifty athletes were expected to compete; the exact number was dependent on how many nations use universality places to enter athletes in addition to the 48 qualifying through time or ranking. 48 athletes from 33 nations competed. The event was won by Steven Gardiner of the Bahamas, with Anthony Zambrano of Colombia taking silver. Those were the first medals in the men's 400 metres for each of those two nations. Kirani James of Grenada won his third consecutive medal in the event with his bronze, making him the first man to earn three medals in the 400 metres.

Anthony José Zambrano de la Cruz is a Colombian sprinter. He won the silver medal at the 2020 Summer Olympics in the 400 metres, setting the new South American record of 43.93 seconds.