2003 World Championships in Athletics – Men's 400 metres

These are the official results of the Men's 400 metres event at the 2003 IAAF World Championships in Paris, France. There were a total number of 51 participating athletes, with seven qualifying heats, three semi-finals and the final held on Tuesday 26 August 2003 at 21:50h. The winning margin was 0.02 seconds which as of May 2024 remains the narrowest winning margin in the men's 400 metres at these championships.

The gold medal was originally won by Jerome Young of the United States in 44.50, but he was later disqualified for doping, together with his teammate Calvin Harrison who finished in sixth place.

Semi-final

 * Held on Sunday 24 August 2003

Heats
Held on Saturday 23 August 2003