The women's 400 metres at the 2012 World Junior Championships in Athletics was held at the Estadi Olímpic Lluís Companys on 11, 12, and 13 July.[1]
| Gold | Ashley Spencer United States | |
| Silver | Kadecia Baird Guyana | |
| Bronze | Erika Rucker United States |
, the existing world junior and championship records were as follows.
| World Junior Record | 49.42 | Tokyo, Japan | 27 August 1991 | ||
| Championship Record | 50.62 | Plovdiv, Bulgaria | 10 August 1990 | ||
| World Junior Leading | 51.02 | Madison, United States | 13 May 2012 | ||
| Broken records during the 2012 World Junior Championships in Athletics | |||||
|---|---|---|---|---|---|
| Championship Record World Junior Leading | 50.50 | Barcelona, Spain | 13 July 2012 | ||
Qualification: first 3 of each heat (Q) plus the 6 fastest times (q) qualified
| Rank | Heat | Lane | Name | Nationality | Time | Note | |
|---|---|---|---|---|---|---|---|
| 1 | 1 | 4 | 52.58 | Q | |||
| 2 | 4 | 9 | 52.71 | Q | |||
| 3 | 4 | 6 | 52.76 | Q, NJ | |||
| 4 | 4 | 7 | 52.86 | Q, SB | |||
| 5 | 1 | 3 | 52.92 | Q, PB | |||
| 6 | 5 | 4 | 52.99 | Q | |||
| 7 | 5 | 6 | 53.29 | Q | |||
| 8 | 6 | 7 | 53.32 | Q | |||
| 9 | 3 | 6 | 53.44 | Q | |||
| 10 | 6 | 8 | 53.53 | Q | |||
| 11 | 1 | 9 | 53.57 | Q, PB | |||
| 12 | 4 | 3 | 53.77 | q, SB | |||
| 13 | 6 | 5 | 53.85 | Q | |||
| 14 | 2 | 6 | 54.02 | Q | |||
| 15 | 4 | 4 | 54.03 | q, PB | |||
| 16 | 5 | 8 | 54.03 | Q, PB | |||
| 17 | 1 | 7 | 54.17 | q, SB | |||
| 18 | 3 | 9 | 54.19 | Q, PB | |||
| 19 | 3 | 8 | 54.34 | Q | |||
| 20 | 2 | 5 | 54.55 | Q | |||
| 21 | 1 | 5 | 54.65 | q, NJ | |||
| 22 | 5 | 7 | 54.67 | q | |||
| 23 | 1 | 6 | 54.70 | q, PB | |||
| 24 | 5 | 9 | 54.73 | PB | |||
| 25 | 4 | 8 | 54.83 | NJ | |||
| 26 | 6 | 4 | 54.88 | ||||
| 27 | 3 | 3 | 55.00 | ||||
| 28 | 2 | 9 | 55.04 | Q | |||
| 29 | 6 | 6 | 55.16 | ||||
| 30 | 3 | 5 | 55.26 | PB | |||
| 31 | 4 | 5 | 55.37 | ||||
| 32 | 2 | 7 | 55.57 | ||||
| 33 | 2 | 8 | 55.96 | ||||
| 34 | 3 | 4 | 56.69 | ||||
| 35 | 6 | 3 | 58.87 | PB | |||
| 36 | 2 | 2 | 1:05.85 | PB | |||
| 37 | 2 | 4 | Reloliza Saimon | 1:09.88 | SB | ||
| – | 5 | 5 | – | DSQ | |||
| – | 5 | 3 | – | DSQ | |||
| – | 6 | 9 | – | DSQ | |||
| – | 2 | 3 | – | DNS | |||
| – | 1 | 8 | – | DNS | |||
| – | 3 | 7 | – | DNS |
Qualification: first 2 of each heat (Q) plus the 2 fastest times (q) qualified
| Rank | Heat | Lane | Name | Nationality | Time | Note | |
|---|---|---|---|---|---|---|---|
| 1 | 1 | 6 | 52.05 | Q, PB | |||
| 2 | 3 | 5 | 52.24 | Q | |||
| 3 | 1 | 4 | 52.40 | Q | |||
| 4 | 1 | 7 | 52.65 | q | |||
| 5 | 1 | 5 | 52.70 | q | |||
| 6 | 3 | 9 | 52.71 | Q, SB | |||
| 7 | 2 | 4 | 52.75 | Q | |||
| 8 | 2 | 6 | 52.87 | Q | |||
| 9 | 3 | 4 | 52.88 | ||||
| 10 | 2 | 7 | 52.97 | ||||
| 11 | 3 | 7 | 53.04 | ||||
| 12 | 1 | 9 | 53.28 | SB | |||
| 13 | 2 | 5 | 53.88 | PB | |||
| 14 | 1 | 8 | 53.90 | PB | |||
| 15 | 3 | 2 | 54.40 | ||||
| 16 | 1 | 2 | 54.42 | NJ | |||
| 17 | 2 | 9 | 54.49 | ||||
| 18 | 1 | 3 | 54.52 | ||||
| 19 | 3 | 8 | 54.67 | ||||
| 20 | 2 | 8 | 54.68 | ||||
| 21 | 2 | 2 | 54.82 | ||||
| 22 | 3 | 6 | 54.98 | ||||
| 23 | 2 | 3 | 55.02 | ||||
| 24 | 3 | 3 | 55.85 |
| Rank | Lane | Name | Nationality | Time | Note | |
|---|---|---|---|---|---|---|
| 6 | 50.50 | CR | ||||
| 4 | 51.04 | AJ | ||||
| 5 | 51.10 | PB | ||||
| 4 | 7 | 51.78 | ||||
| 5 | 3 | 51.87 | NJ | |||
| 6 | 8 | 52.20 | SB | |||
| 7 | 2 | 52.31 | SB | |||
| 8 | 9 | 52.68 | SB |
According to an unofficial count, 40 athletes from 30 countries participated in the event.