It was the first arrest in connection with last month's assassination.
Security officials say the boy told them he was part of a backup squad trained to attack, if the initial attempt on her life had failed.
But local newspapers say investigators are treating the boy's claims with caution.
The unnamed 15-year-old was arrested in Pakistan's North West Frontier Province along with a more senior militant, officials say.
He allegedly told investigators that he would have been "next in line" to attack Ms Bhutto. more
