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".

The Opeongo River is a river in the Saint Lawrence River drainage basin in Nipissing District in northeastern Ontario, Canada. The river is entirely within Algonquin Provincial Park and Opeongo River Provincial Park, except for a small portion around Victoria Lake, and is a left tributary of the Madawaska River.