The women's 400 metres event at the 2004 World Junior Championships in Athletics was held in Grosseto, Italy, at Stadio Olimpico Carlo Zecchini on 13, 14 and 15 July.
| Gold | Natasha Hastings | |
| Silver | Sonita Sutherland | |
| Bronze | Ashlee Kidd |
15 July
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| align=left | align=left | | 52.04 | ||||
| align=left | align=left | | 52.41 | ||||
| align=left | align=left | | 52.45 | ||||
| 4 | align=left | align=left |  | 52.87 | |||
| 5 | align=left | align=left |  | 52.99 | |||
| 6 | align=left | align=left |  | 53.34 | |||
| 7 | align=left | align=left | 53.67 | ||||
| 8 | align=left | align=left |  | 53.82 |
14 July
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 53.56 | Q | ||
| 2 | align=left | align=left | | 53.61 | Q | ||
| 3 | align=left | align=left | 53.93 | Q | |||
| 4 | align=left | align=left | | 54.19 | Q | ||
| 5 | align=left | align=left | 54.49 | ||||
| 6 | align=left | align=left | 56.08 | ||||
| 7 | align=left | align=left | 56.42 | ||||
| 8 | align=left | align=left | 65.15 |
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 53.50 | Q | ||
| 2 | align=left | align=left | | 53.83 | Q | ||
| 3 | align=left | align=left | | 53.93 | Q | ||
| 4 | align=left | align=left | | 54.04 | Q | ||
| 5 | align=left | align=left | 54.24 | ||||
| 6 | align=left | align=left |  | 54.59 | |||
| 7 | align=left | align=left |  | 55.50 | |||
| 8 | align=left | align=left |  | 56.13 |
13 July
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 52.61 | Q | ||
| 2 | align=left | align=left | | 53.37 | Q | ||
| 3 | align=left | align=left | | 53.48 | Q | ||
| 4 | align=left | align=left | 54.32 | q | |||
| 5 | align=left | align=left | | 55.05 | |||
| 6 | align=left | align=left | | 55.50 | |||
| 7 | align=left | align=left | 55.88 | ||||
| 8 | align=left | align=left | | 56.31 |
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 53.56 | Q | ||
| 2 | align=left | align=left | 53.67 | Q | |||
| 3 | align=left | align=left | 54.00 | Q | |||
| 4 | align=left | align=left |  | 54.41 | |||
| 5 | align=left | align=left |  | 55.95 | |||
| 6 | align=left | align=left |  | 56.50 | |||
| 7 | align=left | align=left |  | 57.10 |
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 53.52 | Q | ||
| 2 | align=left | align=left | | 54.05 | Q | ||
| 3 | align=left | align=left | | 54.85 | Q | ||
| 4 | align=left | align=left |  | 54.93 | |||
| 5 | align=left | align=left |  | 55.15 | |||
| 6 | align=left | align=left | | 55.76 |
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | 53.48 | Q | |||
| 2 | align=left | align=left | | 53.54 | Q | ||
| 3 | align=left | align=left | | 53.74 | Q | ||
| 4 | align=left | align=left | 53.78 | q | |||
| 5 | align=left | align=left | 54.25 | q | |||
| 6 | align=left | align=left | | 54.41 | q | ||
| 7 | align=left | align=left | 54.77 | ||||
| 8 | align=left | align=left | 58.21 |
According to an unofficial count, 29 athletes from 24 countries participated in the event.