The men's 400 metres hurdles event at the 2000 World Junior Championships in Athletics was held in Santiago, Chile, at Estadio Nacional Julio Martínez Prádanos on 18, 19 and 21 October.
| Gold | Marek Plawgo | |
| Silver | Pieter de Villiers | |
| Bronze | Ockert Cilliers |
21 October
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| align=left | align=left | | 49.23 | ||||
| align=left | align=left | 50.52 | |||||
| align=left | align=left | 50.58 | |||||
| 4 | align=left | align=left | 50.60 | ||||
| 5 | align=left | align=left |  | 51.05 | |||
| 6 | align=left | align=left |  | 51.48 | |||
| 7 | align=left | align=left |  | 52.26 | |||
| 8 | align=left | align=left |  | 54.95 |
19 October
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 50.40 | Q | ||
| 2 | align=left | align=left | 51.32 | Q | |||
| 3 | align=left | align=left | | 51.76 | Q | ||
| 4 | align=left | align=left | | 51.83 | Q | ||
| 5 | align=left | align=left |  | 51.94 | |||
| 6 | align=left | align=left | 52.50 | ||||
| 7 | align=left | align=left | 52.57 | ||||
| 8 | align=left | align=left |  | 52.81 |
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | 50.21 | Q | |||
| 2 | align=left | align=left | | 50.72 | Q | ||
| 3 | align=left | align=left | 51.10 | Q | |||
| 4 | align=left | align=left | | 51.13 | Q | ||
| 5 | align=left | align=left |  | 51.31 | |||
| 6 | align=left | align=left |  | 51.88 | |||
| 7 | align=left | align=left | | 52.24 | |||
| 8 | align=left | align=left |  | 55.54 |
18 October
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | 50.86 | Q | |||
| 2 | align=left | align=left | 50.90 | Q | |||
| 3 | align=left | align=left | | 51.07 | Q | ||
| 4 | align=left | align=left | | 51.93 | q | ||
| 5 | align=left | align=left | 52.34 | q | |||
| 6 | align=left | align=left | 53.32 | ||||
| 7 | align=left | align=left |  | 54.73 | |||
| align=left | align=left | | DQ | IAAF rule 168.7 |
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 51.95 | Q | ||
| 2 | align=left | align=left | 51.95 | Q | |||
| 3 | align=left | align=left | 52.13 | Q | |||
| 4 | align=left | align=left | | 52.56 | q | ||
| 5 | align=left | align=left |  | 52.72 | |||
| align=left | align=left | DQ | IAAF rule 168.7 | ||||
| align=left | align=left |  | DQ | IAAF rule 168.7 |
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 51.74 | Q | ||
| 2 | align=left | align=left | | 51.85 | Q | ||
| 3 | align=left | align=left | | 52.00 | Q | ||
| 4 | align=left | align=left | | 52.46 | q | ||
| 5 | align=left | align=left |  | 53.08 | |||
| 6 | align=left | align=left |  | 54.87 | |||
| align=left | align=left | | DQ | IAAF rule 168.7 |
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 52.00 | Q | ||
| 2 | align=left | align=left | | 52.36 | Q | ||
| 3 | align=left | align=left | | 52.44 | Q | ||
| 4 | align=left | align=left | | 52.89 | |||
| 5 | align=left | align=left |  | 52.97 | |||
| 6 | align=left | align=left | 54.08 | ||||
| 7 | align=left | align=left |  | 54.76 |
According to an unofficial count, 29 athletes from 23 countries participated in the event.