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.

Steven Gardiner is a Bahamian track and field sprinter competing in the 400 metres and 200 metres. He is the 2020 Olympic and 2019 world champion in the 400 m, and also won the silver medal at the 2017 World Championships in that event. His winning time of 43.48 s from the 2019 World Championships is the Bahamian record and makes him the sixth-fastest man in the history of the event. Gardiner also owns the Bahamian records in the outdoor 300 m and 200 m, with times of 31.83 s and 19.75 s respectively, and the world best in the indoor 300 m at 31.56 s.