The men's 400 metres event at the 2000 Summer Olympics as part of the athletics programme was held at Stadium Australia from 22 to 25 September 2000.

The event was won by 0.56 seconds by Michael Johnson of the United States, successfully defending his 1996 gold medal (the only man to do so in the history of the Olympic 400 metres race).

Gregory Haughton's bronze was Jamaica's first medal in the event since the nation won back-to-back golds in 1948 and 1952.

Five of the finalists from 1996 returned: defending gold medalist Michael Johnson and fourth-place finisher Alvin Harrison of the United States, bronze medalist Davis Kamoga of Uganda, seventh-place finisher Davian Clarke of Jamaica, and eight-place Ibrahim Ismail Muftah of Qatar.

Each National Olympic Committee was permitted to enter up to three athletes that had run 45.80 seconds or faster during the qualification period.