2012 World Junior Championships in Athletics – Men's 400 metres hurdles

The men's 400 metres hurdles at the 2012 World Junior Championships in Athletics was held at the Estadi Olímpic Lluís Companys on 11–13 July.

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

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

Semi-final
Qualification: The first 2 of each heat (Q) and the 2 fastest times (q) qualified

Participation
According to an unofficial count, 52 athletes from 36 countries participated in the event. • 🇦🇺 australia (1)

• 🇦🇹 austria (1)

• 🇧🇭 bahrain (1)

• undefined (2)

• 🇧🇷 brazil (1)

• 🇨🇦 canada (1)

• undefined (1)

• undefined (1)

• 🇫🇮 finland (2)

• 🇩🇪 germany (2)

• undefined (1)

• 🇭🇺 hungary (2)

• 🇮🇳 india (1)

• 🇮🇶 iraq (1)

• 🇮🇪 ireland (1)

• 🇮🇹 italy (2)

• 🇯🇲 jamaica (2)

• 🇯🇵 japan (2)

• 🇰🇪 kenya (2)

• 🇲🇽 mexico (2)

• 🇳🇿 new zealand (1)

• 🇳🇴 norway (2)

• 🇵🇱 poland (1)

• undefined (1)

• 🇶🇦 qatar (1)

• 🇷🇺 russia (2)

• undefined (2)

• undefined (1)

• undefined (2)

• 🇰🇷 south korea (1)

• 🇪🇸 spain (2)

• undefined (1)

• 🇹🇳 tunisia (1)

• 🇺🇦 ukraine (1)

• 🇺🇸 united states (2)

• 🇻🇪 venezuela (2)