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.

[1][3][4] It is the largest lake in Algonquin Provincial Park[5] and the source of the Opeongo River.

The lake's name comes from the Algonquian word opeauwingauk meaning "sandy narrows".

A store with camping supplies and dock, Algonquin access point 11[6] and the Harkness Laboratory of Fisheries Research[5] are located on Sproule Bay at south end of South Arm and are all accessible from Ontario Highway 60.

Consequently, the lake is a popular starting point for canoe trips into the interior of the park.