The men's 400 metres was an event at the 1984 Summer Olympics in Los Angeles, California. It was held from August 4 to August 8. Eighty athletes from 56 nations competed. The maximum number of athletes per nation had been set at 3 since the 1930 Olympic Congress. The event was won by Alonzo Babers, returning the United States to the top of the podium for the first time since 1972. Gabriel Tiacoh won the Ivory Coast's first Olympic medal in any event, with a silver.

At the 1984 Summer Olympics in Los Angeles, 41 events in athletics were contested. There were a total number of 1273 participating athletes from 124 countries. Women's marathon, women's 3000 meters, and women's 400 meters hurdles debuted at these Games.