The men's 400 metres hurdles at the 2021 World Athletics U20 Championships was held at the Kasarani Stadium on 19, 21 and 22 August.

[1] Qualification: First 4 of each heat (Q) and the 4 fastest times (q) qualified for the semifinals.

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

[3] The final was held on 22 August at 15:00.

Swedish athlete Oskar Edlund crossed the line first in a time of 49.20 seconds, but was later disqualified for pulling one of his legs outside his lane to clear a hurdle.