Alcester is a city in northern Union County, South Dakota, United States.

[5] Alcester was platted in 1879; early variant names were Linia and Irene.

[6] The present name honors Colonel Alcester of the British army.

[8] According to the United States Census Bureau, the city has a total area of 0.33 square miles (0.85 km2), all land.

The racial makeup of the city was 98.41% White, 0.23% Native American, 0.91% Asian, and 0.45% from two or more races.

About 6.5% of families and 7.9% of the population were below the poverty line, including 3.8% of those under the age of 18 and 16.0% of those 65 and older.