RCMP say search teams had been looking for the child since he was reported missing at noon on Tuesday.
He was found four hours later by an area resident walking near the river and rushed to hospital, but was pronounced dead.
RCMP say the boy and his six-year-old brother had been playing near the water earlier in the day at a relative's property about eight kilometres south of the town of Spirit River.
The younger boy slipped into the water and was not able to get back to shore.
He was found about a kilometre downstream.
Sgt. Ryan Dlin, Spirit River Detachment: "We are deeply saddened by this tragic loss and we are keeping the family and community in our prayers," said Sgt. Ryan Dlin of the Spirit River detachment.
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