Castlewood is a city in Hamlin County, South Dakota, United States. It is part of the Watertown, South Dakota Micropolitan Statistical Area. The population was 698 at the 2020 census. and was estimated to be 707 in 2022.

Hamlin County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 6,164. Its county seat is Hayti. The county was created in 1873 and organized in 1878. It was named for Hannibal Hamlin, Lincoln's first vice-president.