Emergency crews were called to the shore of the Ottawa River at Bate Island, near the Champlain Bridge on Saturday night just before 7 p.m. after reports of a water rescue.
Two men, aged 39-years-old and 24-years-old, went into the river to rescue a 12-year-old boy.
While the 24-year-old was able to rescue the boy and return to shore, the other man did not return.
The boy was treated for hypothermia.
Police along with the Ottawa Fire Water Rescue team are scouring the river for the other man.