Frederick is a town in northwestern Brown County, South Dakota, United States. The population was 215 at the 2020 census. Home to the annual Frederick Finn Fest, the town is part of the Aberdeen Micropolitan Statistical Area.

Brown County is a county in the U.S. state of South Dakota. As of the 2020 census, the population was 38,301, making it the fourth most populous county in South Dakota. Its county seat is Aberdeen. The county is named for Alfred Brown, of Hutchinson County, South Dakota, a Dakota Territory legislator in 1879.