Firefighters responded to the Co Lam Pagoda in the 3500 block of South Graham Street after receiving reports of a fire and explosion in the lower level of the building.
Crews managed to knock down the fire quickly, keeping it contained to one room.
Medics transported the boy to Harborview Medical Center with first-, second- and third-degree burns to his hands and face.
Detectives from Seattle's Arson/Bomb Squad believe the explosion and fire were the result of the boy and several friends playing with loose pyrotechnic powder found in the building's storage room.
Members of the bomb squad carted away stacks of fireworks from the scene.
Damage to the property is estimated at $10,000.
Detectives continue to investigate the incident.