Lesser Slave Lake, Natural lake in northern Alberta, Canada

Lesser Slave Lake is a lake in central Alberta, about 250 km northwest of Edmonton, roughly 97–100 km long and up to 15–18 km wide, with a surface of about 1,160–1,168 km² and a maximum depth of 20.5 m. It drains east through the Lesser Slave River into the Athabasca River. The town of Slave Lake sits at the eastern outflow; Highway 2 and the Canadian National Railway run along the south shore.

The name derives from the Slavey (Dene) people, not from slavery and not from Cree; details of the naming are treated in the article on Great Slave Lake. The lake lies in the Big Lakes and Lesser Slave River municipal districts, about 250 km northwest of Edmonton at 577 m above sea level. It drains east through the Lesser Slave River into the Athabasca River, part of the Mackenzie system reaching the Arctic Ocean.

The lake's name refers to the Slavey people, not to slavery. Several First Nations reserves line the shore, among them Kapawe'no, Sucker Creek, Driftpile, Swan River and Sawridge. The Cree name recorded for the lake is Ayahchiyiniw Sākahikan.

Highway 2 and the Canadian National Railway run along the southern shore, and the Bicentennial Highway ends at the lake's eastern tip, where the town of Slave Lake sits by the outflow of the Lesser Slave River. The entire north shore is protected, with camping along sand and some rocky beaches in Lesser Slave Lake Provincial Park; Hilliard's Bay Provincial Park, the Wildland and Grouard Trail Park Reserve are other reserves. Fishing is legal.

The lake lies on a major flyway for migrating birds, which draws birders to its shores; the provincial park on the south side has lakeside camping along sand beaches, with some rocky stretches. The whole north shore is protected, including Hilliard's Bay Provincial Park, the Lesser Slave Lake Wildland and Grouard Trail Park Reserve.