The men's 400 metres event at the 2017 European Athletics U23 Championships was held in Bydgoszcz, Poland, at Zdzisław Krzyszkowiak Stadium on 13, 14 and 15 July.
| Gold | Luka Janežič | |
| Silver | Karsten Warholm | |
| Bronze | Benjamin Lobo Vedel |
14 July[1]
Qualification rule: First 3 (Q) and the next 4 fastest (q) qualified for the semifinals.
| Rank | Heat | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|---|
| 1 | 1 | align=left | align=left | 46.44 | Q | |||
| 2 | 4 | align=left | align=left | 46.70 | Q | |||
| 3 | 4 | align=left | align=left | 46.75 | Q, =PB | |||
| 4 | 1 | align=left | align=left | 46.76 | Q | |||
| 5 | 4 | align=left | align=left | 46.80 | Q | |||
| 6 | 2 | align=left | align=left | 46.82 | Q | |||
| 7 | 4 | align=left | align=left | 46.82 | q | |||
| 8 | 1 | align=left | align=left | 46.87 | Q | |||
| 9 | 1 | align=left | align=left | 46.88 | q, PB | |||
| 10 | 4 | align=left | align=left | 47.09 | q, PB | |||
| 11 | 3 | align=left | align=left | 47.19 | Q | |||
| 12 | 2 | align=left | align=left | 47.25 | Q | |||
| 13 | 4 | align=left | align=left | 47.38 | q | |||
| 14 | 1 | align=left | align=left | 47.44 | ||||
| 15 | 3 | align=left | align=left | 47.47 | Q | |||
| 16 | 1 | align=left | align=left | 47.47 | ||||
| 17 | 2 | align=left | align=left | 47.50 | Q | |||
| 18 | 3 | align=left | align=left | 47.66 | Q | |||
| 19 | 1 | align=left | align=left | 47.63 | ||||
| 20 | 4 | align=left | align=left | 47.65 | ||||
| 21 | 2 | align=left | align=left | 47.79 | ||||
| 22 | 3 | align=left | align=left | 48.04 | ||||
| 23 | 2 | align=left | align=left | 48.05 | ||||
| 24 | 3 | align=left | align=left | 48.13 | ||||
| 25 | 2 | align=left | align=left | 48.17 | ||||
| 26 | 3 | align=left | align=left | 48.22 | ||||
| 27 | 3 | align=left | align=left | 48.26 | ||||
| 28 | 2 | align=left | align=left | 48.32 | ||||
| 29 | 3 | align=left | align=left | 48.52 | ||||
| 30 | 2 | align=left | align=left | 50.86 | ||||
| 31 | 1 | align=left | align=left | 53.00 |
14 July[2]
Qualification rule: First 3 (Q) and the next 2 fastest (q) qualified for the final.
| Rank | Heat | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|---|
| 1 | 2 | align=left | align=left | 45.73 | Q | |||
| 2 | 2 | align=left | align=left | 45.85 | Q, NU23 | |||
| 3 | 2 | align=left | align=left | 45.98 | Q, PB | |||
| 4 | 1 | align=left | align=left | 46.06 | Q | |||
| 5 | 1 | align=left | align=left | 46.20 | Q | |||
| 6 | 1 | align=left | align=left | 46.32 | Q | |||
| 7 | 2 | align=left | align=left | 46.33 | q, PB | |||
| 8 | 1 | align=left | align=left | 46.39 | q, PB | |||
| 9 | 2 | align=left | align=left | 46.49 | ||||
| 10 | 2 | align=left | align=left | 46.56 | ||||
| 11 | 1 | align=left | align=left | 46.60 | ||||
| 12 | 1 | align=left | align=left | 46.74 | ||||
| 13 | 1 | align=left | align=left | 46.93 | PB | |||
| 14 | 1 | align=left | align=left | 47.06 | ||||
| 15 | 2 | align=left | align=left | 47.23 | ||||
| 16 | 2 | align=left | align=left | 47.32 | =SB |
15 July[3]
| Rank | Lane | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|---|
| 6 | align=left | align=left | 45.33 | |||||
| 4 | align=left | align=left | 45.75 | |||||
| 7 | align=left | align=left | 46.08 | |||||
| 4 | 8 | align=left | align=left | 46.29 | ||||
| 5 | 5 | align=left | align=left | 46.51 | ||||
| 6 | 3 | align=left | align=left | 46.79 | ||||
| 7 | 9 | align=left | align=left | 46.80 | ||||
| 8 | 2 | align=left | align=left | 46.92 |