Emergency services were called to a home on McDougall Street, Casino, following reports of a stabbing around 1:20 p.m.
A child in the house was found with stab wounds. He was treated by police and paramedics, but died at the scene.
He has not yet been formally identified, but is believed to be 17 years old.
Initial investigations suggest the stabbing was a target, police said.
The investigation is ongoing.