The men's 800 metres event at the 2004 World Junior Championships in Athletics was held in Grosseto, Italy, at Stadio Olimpico Carlo Zecchini on 13, 14 and 16 July.
| Gold | Majid Saeed Sultan | |
| Silver | Alfred Kirwa Yego | |
| Bronze | Selahattin Çobanoğlu |
16 July
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| align=left | align=left | | 1:47.33 | ||||
| align=left | align=left | | 1:47.39 | ||||
| align=left | align=left | | 1:47.71 | ||||
| 4 | align=left | align=left |  | 1:48.37 | |||
| 5 | align=left | align=left | | 1:48.76 | |||
| 6 | align=left | align=left | | 1:49.08 | |||
| 7 | align=left | align=left | 1:49.85 | ||||
| 8 | align=left | align=left | 1:50.59 |
14 July
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 1:49.70 | Q | ||
| 2 | align=left | align=left | | 1:49.86 | Q | ||
| 3 | align=left | align=left | | 1:49.98 | Q | ||
| 4 | align=left | align=left | 1:50.05 | q | |||
| 5 | align=left | align=left |  | 1:50.41 | |||
| 6 | align=left | align=left |  | 1:50.51 | |||
| 7 | align=left | align=left |  | 1:51.18 | |||
| 8 | align=left | align=left |  | 1:51.97 |
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 1:49.02 | Q | ||
| 2 | align=left | align=left | | 1:49.53 | Q | ||
| 3 | align=left | align=left | 1:49.53 | Q | |||
| 4 | align=left | align=left | | 1:49.58 | q | ||
| 5 | align=left | align=left | | 1:50.63 | |||
| 6 | align=left | align=left |  | 1:50.79 | |||
| 7 | align=left | align=left |  | 1:50.97 | |||
| 8 | align=left | align=left | 1:51.30 |
13 July
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 1:48.96 | Q | ||
| 2 | align=left | align=left | 1:49.31 | Q | |||
| 3 | align=left | align=left | | 1:49.34 | q | ||
| 4 | align=left | align=left | | 1:50.44 | q | ||
| 5 | align=left | align=left |  | 1:50.98 | |||
| 6 | align=left | align=left |  | 2:01.92 |
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 1:49.50 | Q | ||
| 2 | align=left | align=left | | 1:49.88 | Q | ||
| 3 | align=left | align=left | | 1:49.93 | q | ||
| 4 | align=left | align=left | | 1:50.75 | q | ||
| 5 | align=left | align=left | 1:51.31 |
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 1:51.77 | Q | ||
| 2 | align=left | align=left | 1:51.96 | Q | |||
| 3 | align=left | align=left |  | 1:52.85 | |||
| 4 | align=left | align=left | 1:52.94 | ||||
| 5 | align=left | align=left |  | 1:53.74 | |||
| 6 | align=left | align=left |  | 1:54.88 | |||
| 7 | align=left | align=left |  | 2:03.53 |
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 1:50.98 | Q | ||
| 2 | align=left | align=left | | 1:51.30 | Q | ||
| 3 | align=left | align=left | | 1:51.51 | |||
| 4 | align=left | align=left | | 1:51.64 | |||
| 5 | align=left | align=left |  | 1:52.42 | |||
| 6 | align=left | align=left | | 1:53.18 | |||
| 7 | align=left | align=left | 2:08.65 |
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 1:49.19 | Q | ||
| 2 | align=left | align=left | | 1:49.58 | Q | ||
| 3 | align=left | align=left | 1:49.64 | q | |||
| 4 | align=left | align=left | | 1:50.72 | q | ||
| 5 | align=left | align=left |  | 1:50.89 | |||
| 6 | align=left | align=left |  | 1:56.63 |
According to an unofficial count, 31 athletes from 23 countries participated in the event.