The women's 400 metres event at the 1986 World Junior Championships in Athletics was held in Athens, Greece, at Olympic Stadium on 16, 17 and 18 July.
| Gold | Susanne Sieger | |
| Silver | Olga Pesnopevtseva | |
| Bronze | Sandie Richards |
18 July
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| align=left | align=left | 52.02 | |||||
| align=left | align=left | 52.17 | |||||
| align=left | align=left | | 52.23 | ||||
| 4 | align=left | align=left |  | 52.25 | |||
| 5 | align=left | align=left | | 53.21 | |||
| 6 | align=left | align=left |  | 53.57 | |||
| 7 | align=left | align=left |  | 54.18 | |||
| 8 | align=left | align=left |  | 54.55 |
17 July
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | 52.40 | Q | |||
| 2 | align=left | align=left | | 53.38 | Q | ||
| 3 | align=left | align=left | | 53.71 | Q | ||
| 4 | align=left | align=left | | 53.87 | Q | ||
| 5 | align=left | align=left |  | 54.84 | |||
| 6 | align=left | align=left |  | 55.04 | |||
| 7 | align=left | align=left |  | 55.17 | |||
| 8 | align=left | align=left |  | 55.19 |
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 52.53 | Q | ||
| 2 | align=left | align=left | 52.82 | Q | |||
| 3 | align=left | align=left | | 53.93 | Q | ||
| 4 | align=left | align=left | | 54.70 | Q | ||
| 5 | align=left | align=left | 55.20 | ||||
| 6 | align=left | align=left | 56.57 | ||||
| 7 | align=left | align=left | | 56.92 | |||
| 8 | align=left | align=left | | 57.31 |
16 July
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 54.38 | Q | ||
| 2 | align=left | align=left | | 54.58 | Q | ||
| 3 | align=left | align=left | | 54.77 | Q | ||
| 4 | align=left | align=left | 55.26 | ||||
| 5 | align=left | align=left |  | 56.37 | |||
| 6 | align=left | align=left | 57.59 | ||||
| 7 | align=left | align=left | 58.20 |
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | 52.97 | Q | |||
| 2 | align=left | align=left | 54.75 | Q | |||
| 3 | align=left | align=left | | 54.76 | Q | ||
| 4 | align=left | align=left | | 54.92 | q | ||
| 5 | align=left | align=left | 55.27 |
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 55.23 | Q | ||
| 2 | align=left | align=left | | 55.86 | Q | ||
| 3 | align=left | align=left | | 55.97 | Q | ||
| 4 | align=left | align=left | 56.61 | ||||
| 5 | align=left | align=left | 56.83 | ||||
| 6 | align=left | align=left |  | 56.93 |
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | | 53.25 | Q | ||
| 2 | align=left | align=left | | 54.89 | Q | ||
| 3 | align=left | align=left | | 56.20 | Q | ||
| 4 | align=left | align=left | 56.50 | ||||
| 5 | align=left | align=left |  | 59.20 | |||
| 6 | align=left | align=left | | 60.70 | |||
| 7 | align=left | align=left |  | 62.82 |
| Rank | Name | Nationality | Time | Notes | |||
|---|---|---|---|---|---|---|---|
| 1 | align=left | align=left | 52.73 | Q | |||
| 2 | align=left | align=left | | 53.41 | Q | ||
| 3 | align=left | align=left | 54.90 | Q | |||
| 4 | align=left | align=left |  | 55.76 | |||
| 5 | align=left | align=left | 57.46 | ||||
| 6 | align=left | align=left |  | 58.13 | |||
| 7 | align=left | align=left |  | 60.06 | |||
| 8 | align=left | align=left |  | 65.32 |
According to an unofficial count, 33 athletes from 27 countries participated in the event.