2021 World Athletics U20 Championships – Men's 400 metres

The men's 400 metres at the 2021 World Athletics U20 Championships was held at the Kasarani Stadium 18 and 21 August.

Heats
Qualification: First 2 of each heat (Q) and the 2 fastest times (q) qualified for the final.

Final
The final was held on 21 August at 17:19.