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.

Algonquin Provincial Park is an Ontario provincial park located between Georgian Bay and the Ottawa River, mostly within the Unorganized South Part of Nipissing District. Established in 1893, it is the oldest provincial park in Canada. Additions since its creation have increased the park to its current size of about 7,653 km2 (2,955 sq mi). The park is contiguous with several smaller, administratively separate provincial parks that protect important rivers in the area, resulting in a larger total protected area.