Selby is a city in Walworth County, South Dakota, United States. The population was 642 at the 2010 census. It is the county seat of Walworth County.
Selby is located at 45°30′20″N 100°1′53″W / 45.50556°N 100.03139°W / 45.50556; -100.03139 (45.505692, -100.031369).
According to the United States Census Bureau, the city has a total area of 0.8 square miles (2.1 km²), all of it land.