The boys' 400 metres at the 2015 World Youth Championships in Athletics was held at the Estadio Olímpico Pascual Guerrero in Cali, Colombia from 15 to 17 July 2015.

[1] Christopher Taylor of Jamaica won the event.

[2] Prior to the competition, the following records were as follows.

First 3 in each heat (Q) and the next 6 fastest (q) advance to the semifinals.

[3] First 2 in each heat (Q) and the next 2 fastest (q) advance to the final.