The men's 4 × 100 metres relay at the 2021 World Athletics U20 Championships was held at the Kasarani Stadium on 21 and 22 August.

[1] Qualification: First 3 of each heat ( Q ) plus the 2 fastest times ( q ) qualified for the final.

[2][3] The final was held on 22 August at 16:46.

[4]