The men's 400 metres at the 2024 World Athletics U20 Championships was held at the Estadio Atlético de la VIDENA in Lima, Peru on 28 and 29 August 2024.

U20 standing records prior to the 2024 World Athletics U20 Championships were as follows:[1] The first 3 athletes in each heat (Q) and the next 3 fastest (q) qualified to the semi-finals.

[1] [2] [2] [2] [2] [2] [2] [2] The first 2 athletes in each heat (Q) and the next 2 fastest (q) qualified to the semi-final.

[3] [4] [4] [4] [5]