The women's 400 metres event at the 2017 European Athletics U23 Championships was held in Bydgoszcz, Poland, at Zdzisław Krzyszkowiak Stadium on 14 and 15 July.
| Gold | Gunta Latiševa-Čudare | |
| Silver | Laura Müller | |
| Bronze | Laura de Witte |
14 July[1]
Qualification rule: First 2 (Q) and the next 2 fastest (q) qualified for the final.
| Rank | Heat | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|---|
| 1 | 1 | align=left | align=left | 51.80 | Q, EL | |||
| 2 | 2 | align=left | align=left | 51.94 | Q, NR | |||
| 3 | 1 | align=left | align=left | 52.15 | Q, NU23R | |||
| 4 | 1 | align=left | align=left | 52.32 | q | |||
| 5 | 3 | align=left | align=left | 52.80 | Q | |||
| 6 | 2 | align=left | align=left | 52.81 | Q, PB | |||
| 7 | 2 | align=left | align=left | 52.88 | q, PB | |||
| 8 | 2 | align=left | align=left | 52.90 | ||||
| 9 | 3 | align=left | align=left | 52.96 | Q | |||
| 10 | 3 | align=left | align=left | 53.07 | PB | |||
| 11 | 2 | align=left | align=left | 53.22 | PB | |||
| 12 | 3 | align=left | align=left | 53.25 | ||||
| 13 | 1 | align=left | align=left | 53.47 | PB | |||
| 14 | 3 | align=left | align=left | 53.58 | ||||
| 15 | 1 | align=left | align=left | 53.78 | ||||
| 16 | 2 | align=left | align=left | 53.94 | SB | |||
| 17 | 1 | align=left | align=left | 54.02 | ||||
| 18 | 2 | align=left | align=left | 54.06 | SB | |||
| 19 | 3 | align=left | align=left | 54.20 | ||||
| 20 | 1 | align=left | align=left | 54.34 | ||||
| 21 | 2 | align=left | align=left | 54.44 | ||||
| 22 | 3 | align=left | align=left | 54.59 | ||||
| 23 | 1 | align=left | align=left | 54.99 | ||||
| 24 | 3 | align=left | align=left | 54.99 | ||||
| 25 | 3 | align=left | align=left | 57.53 | SB |
15 July[2]
| Rank | Lane | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|---|
| 4 | align=left | align=left | 52.00 | |||||
| 3 | align=left | align=left | 52.42 | |||||
| 5 | align=left | align=left | 52.51 | |||||
| 4 | 7 | align=left | align=left | 53.06 | ||||
| 5 | 6 | align=left | align=left | 53.18 | ||||
| 6 | 2 | align=left | align=left | 53.62 | ||||
| 7 | 9 | align=left | align=left | 54.06 | ||||
| 8 | 8 | align=left | align=left | 54.25 |