Officials said the boy died at Cascade Valley Hospital on Sunday. His name has not been released.
Sherrif's office officials said deputies were called when the boy's family noticed he was missing.
The boy had been playing in the river at Twin Rivers Park with other kids and was not wearing a flotation device, officials said.
A crew in a sheriff's office helicopter spotted the boy underwater about 1/4 mile down the river from where he was last seen playing, and crews on the ground brought him to medics on shore.
Lt. Rodney Rochon of the sheriff's office Marine Services Unit said the boy's death is the eighth drowning this year Snohomish County and the second in the Stillaguamish River.