2012 World Junior Championships in Athletics – Women's 400 metres

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.

Prior to the competition^[update], the existing world junior and championship records were as follows.

Qualification: first 3 of each heat (Q) plus the 6 fastest times (q) qualified

Qualification: first 2 of each heat (Q) plus the 2 fastest times (q) qualified

According to an unofficial count, 40 athletes from 30 countries participated in the event.