Lake Koshkonong is a census-designated place (CDP) in the towns of Koshkonong and Sumner, Jefferson County, Wisconsin, United States.

As of the census[2] of 2000, there were 1,219 people, 492 households, and 338 families residing in the CDP.

The population density was 85.2 people per square mile (32.9/km2).

There were 492 households, out of which 25.4% had children under the age of 18 living with them, 59.8% were married couples living together, 4.7% had a female householder with no husband present, and 31.1% were non-families.

22.6% of all households were made up of individuals, and 9.8% had someone living alone who was 65 years of age or older.