The men's 800 metres at the 2021 World Athletics U20 Championships was held at the Kasarani Stadium on 19, 21 and 22 August.

[1] Qualification: First 3 of each heat (Q) and the 4 fastest times (q) qualified for the semifinals.

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

[3] The final was held on 22 August at 15:31.

[4]