Alcester is a city in Union County, South Dakota, United States. It is part of the Sioux City, IA–NE–SD Metropolitan Statistical Area. The population was 807 at the 2010 census.
Before incorporation, the settlement's name was Linia before being changed to Irene. In 1879, the Western Town Lot Company changed its name to Alcester in honor of Colonel Alcester of the British army.
According to the United States Census Bureau, the city has a total area of 0.3 square mile (0.9 km²), all land.