The men's 400 metres was a track & field athletics event at the 1900 Summer Olympics in Paris. It was held on July 14, and July 15, 1900. The races were held on a track of 500 metres in circumference. 15 athletes from six nations competed.

The men's 400 metres race was the second-shortest of the flat-track events on the Athletics at the 1896 Summer Olympics programme. The competition's preliminary round was the last held on the first day, 6 April. The competitors were split into two groups. The top two runners in each heat advanced to the final, which was held on the second day, 7 April.