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.

Records
, the existing world junior and championship records were as follows.

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

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

Participation
According to an unofficial count, 40 athletes from 30 countries participated in the event. • 🇦🇺 australia (2)

• 🇦🇹 austria (2)

• undefined (2)

• undefined (1)

• undefined (1)

• 🇨🇦 canada (1)

• 🇨🇳 china (2)

• DR Congo (1)

• undefined (1)

• 🇫🇮 finland (1)

• 🇩🇪 germany (1)

• undefined (1)

• 🇬🇾 guyana (1)

• 🇭🇺 hungary (1)

• 🇮🇪 ireland (1)

• 🇮🇹 italy (1)

• 🇯🇲 jamaica (2)

• 🇰🇿 kazakhstan (1)

• undefined (1)

• 🇲🇽 mexico (1)

• 🇫🇲 micronesia (1)

• 🇵🇱 poland (1)

• undefined (1)

• undefined (1)

• 🇷🇴 romania (2)

• 🇷🇺 russia (2)

• undefined (2)

• undefined (1)

• 🇺🇦 ukraine (2)

• 🇺🇸 united states (2)