The men's 400 metres hurdles event at the 2004 World Junior Championships in Athletics was held in Grosseto, Italy, at Stadio Olimpico Carlo Zecchini on 14, 15 and 16 July.
| Gold | Kerron Clement | |
| Silver | Brandon Johnson | |
| Bronze | Ibrahim Al-Hamaidi |
16 July
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| align=left | align=left | | 48.51 | ||||
| align=left | align=left | | 48.62 | ||||
| align=left | align=left | 48.94 | |||||
| 4 | align=left | align=left | 49.06 | ||||
| 5 | align=left | align=left |  | 50.24 | |||
| 6 | align=left | align=left | 51.17 | ||||
| 7 | align=left | align=left | 51.59 | ||||
| 8 | align=left | align=left |  | 52.07 |
15 July
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | 49.55 | Q | |||
| 2 | align=left | align=left | 50.20 | Q | |||
| 3 | align=left | align=left | | 50.85 | Q | ||
| 4 | align=left | align=left | | 50.96 | Q | ||
| 5 | align=left | align=left |  | 51.69 | |||
| 6 | align=left | align=left |  | 52.21 | |||
| 7 | align=left | align=left |  | 52.89 | |||
| align=left | align=left |  | DQ | IAAF rule 168.7 |
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 49.01 | Q | ||
| 2 | align=left | align=left | | 49.78 | Q | ||
| 3 | align=left | align=left | 50.11 | Q | |||
| 4 | align=left | align=left | 50.32 | Q | |||
| 5 | align=left | align=left |  | 50.68 | |||
| 6 | align=left | align=left |  | 51.87 | |||
| 7 | align=left | align=left | 51.92 | ||||
| 8 | align=left | align=left |  | 52.20 |
14 July
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 51.27 | Q | ||
| 2 | align=left | align=left | 51.30 | Q | |||
| 3 | align=left | align=left | | 51.46 | Q | ||
| 4 | align=left | align=left | | 51.47 | q | ||
| 5 | align=left | align=left | | 52.03 | q | ||
| 6 | align=left | align=left |  | 52.15 | |||
| 7 | align=left | align=left | | 52.24 | |||
| 8 | align=left | align=left | 53.66 |
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | 50.02 | Q | |||
| 2 | align=left | align=left | 51.12 | Q | |||
| 3 | align=left | align=left | | 51.56 | Q | ||
| 4 | align=left | align=left | | 52.01 | q | ||
| 5 | align=left | align=left | 52.86 | ||||
| 6 | align=left | align=left |  | 52.97 | |||
| 7 | align=left | align=left | | 53.05 | |||
| 8 | align=left | align=left |  | 53.48 |
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 50.10 | Q | ||
| 2 | align=left | align=left | 51.08 | Q | |||
| 3 | align=left | align=left | | 51.48 | Q | ||
| 4 | align=left | align=left | 51.63 | q | |||
| 5 | align=left | align=left | | 52.74 | |||
| 6 | align=left | align=left |  | 54.27 | |||
| 7 | align=left | align=left | 55.26 |
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 50.11 | Q | ||
| 2 | align=left | align=left | | 51.96 | Q | ||
| 3 | align=left | align=left | | 52.02 | Q | ||
| 4 | align=left | align=left | 52.47 | ||||
| 5 | align=left | align=left | 52.82 | ||||
| 6 | align=left | align=left |  | 53.09 | |||
| 7 | align=left | align=left | 53.28 |
According to an unofficial count, 30 athletes from 22 countries participated in the event.