Lake Couchiching, Lake in Central Ontario, Canada

Lake Couchiching is a lake in Central Ontario, Canada, 16 kilometres long and slightly less than 5 kilometres wide. It is separated from Lake Simcoe by a narrow channel, on which the city of Orillia sits. The Trent–Severn Waterway enters Lake Simcoe by the Talbot River and exits Couchiching by the Severn River, which empties into Georgian Bay.

The lake lies on the Trent-Severn Waterway, which enters Lake Simcoe by the Talbot River, passes through the narrow channel at Orillia and leaves Couchiching by the Severn River, emptying into Georgian Bay. Orillia, on that channel, is the main settlement on the lake. Stephen Leacock kept a summer house near Orillia, now a national historic site museum.

The name comes from the Ojibwe gojijiing, "inlet". Orillia-born singer-songwriter Gordon Lightfoot paid homage to the lake in his song "Couchiching", and a statue of him stands in a park above the lake and at a town intersection. The Couchiching Institute on Public Affairs holds its annual conference on the shore every August.

The lake offers activities throughout the year, from boating and fishing in summer to ice fishing from the Orillia shoreline during winter. Visitors can access the water from several locations along the shore, each offering different conditions depending on the season.

The Trent-Severn Waterway enters Lake Simcoe by the Talbot River and leaves Couchiching by the Severn River, which flows into Georgian Bay. A 2012 study recorded a microalgae density of 2.4 × 10^7/cm² with high species diversity, plus low total phosphorus and high organic carbon compared with Lake Simcoe.