Codington County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 28,325, making it the 7th most populous county in South Dakota. Its county seat is Watertown. The county was created in 1877 and organized in 1878. It is named for Rev. George S. S. Codington, Dakota Territory legislator. Codington County comprises the Watertown, SD Micropolitan Statistical Area.