Lake Winnipegosis is a large (5,370 km²) lake in central North America, in Manitoba, Canada, some 300 km northwest of Winnipeg. It is Canada's eleventh-largest lake. An alternate spelling, once common but now rare, is Lake Winipigoos or simply 'Lake Winipigis'. The elongated, 240 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 Waterhen 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. Winnipeg means 'big muddy waters' and Winnipegosis means 'little muddy waters'.
The lake is famous for its commercial fishery of walleye and other freshwater species. Northern pike and mullet together now account for over 80 percent of its commercial fishing. It is also well known for its migratory bird populations, which make it a prime hunting area in the fall.