Sumner is a town in Jefferson County, Wisconsin, United States.

[3] 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.

The racial makeup of the town was 98.89% White, 0.33% Native American, 0.22% Asian, 0.11% Pacific Islander, and 0.44% from two or more races.

23.2% of all households were made up of individuals, and 13.5% had someone living alone who was 65 years of age or older.