2021 European Athletics U23 Championships – Men's 400 metres

The men's 400 metres event at the 2021 European Athletics U23 Championships was held in Tallinn, Estonia, at Kadriorg Stadium on 8, 9 and 10 July.

Records
Prior to the competition, the records were as follows:

Round 1
Qualification rule: First 4 in each heat (Q) and the next 4 fastest (q) advance to the Semi-Finals.

Semifinals
Qualification rule: First 2 in each heat (Q) and the next 2 fastest (q) advance to the Final.