Opeongo Lake is a lake in the Saint Lawrence River drainage basin in the geographic townships of Bower, Dickson, Preston and Sproule in the Unorganized South Part of Nipissing District in Northeastern Ontario, Canada. It is the largest lake in Algonquin Provincial Park and the source of the Opeongo River. The lake's name comes from the Algonquian word opeauwingauk meaning "sandy narrows".
It has three arms, North, East and South, joined by narrows into a Y shape. The total area is , the average depth and the maximum depth . and the Harkness Laboratory of Fisheries Research