Woonsocket is a city in Sanborn County, South Dakota, United States. The population was 631 at the 2020 census. It is the county seat of Sanborn County. Woonsocket was developed in 1883 as a railroad town because of its location at the junction on the Chicago, Milwaukee and Saint Paul Railroad. C.H. Prior, the superintendent of the railroad, named the town Woonsocket after his home town of Woonsocket, Rhode Island. The town was incorporated in 1888.