The elongated, 195 kilometre long lake is the second-largest of three large lakes in central Manitoba; the other two are Lake Winnipeg, the largest, and Lake Manitoba. All three lakes are on the floor of the prehistoric Glacial Lake Agassiz.

The lake's watershed extends over some 49,825 km² in Manitoba and Saskatchewan. Tributaries include the Red Deer, Woody, and Swan rivers. The lake drains through the Water Hen River into Lake Manitoba, and is thus part of the Lake Winnipeg, Nelson River, and Hudson Bay watersheds.

The lake's name derives from that of Lake Winnipeg, with a diminutive suffix.