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.

The United States was the host nation of the 1984 Summer Olympics in Los Angeles, California. It was the nineteenth time that Team USA participated, having boycotted the 1980 Summer Olympics. 522 competitors, 339 men and 183 women, took part in 217 events in 25 sports.