Sumner is a town in Jefferson County, Wisconsin, United States. The population was 832 at the 2010 census. The census-designated place of Lake Koshkonong and the unincorporated communities of Busseyville, Carcajou, Glenn Oaks Beach, Koshkonong Manor, and North Shore are located within the town. The community of Blackhawk Island is located partially in the town.
Geography
According to the United States Census Bureau, the town has a total area of 31.3 square miles (81.0 km<sup>2</sup>), of which 16.3 square miles (42.2 km<sup>2</sup>) is land and 15.0 square miles (38.9 km<sup>2</sup>), or 47.97%, is water.
Demographics
As of the census