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

[5] It is part of the Sioux City, IA-NE-SD Metropolitan Statistical Area.

[8] During the winter of 1880–1, the town was ravaged by a smallpox epidemic, reported to have arrived in the area via Russian Mennonite emigrants en route to Yankton, in the Dakota Territory.

The town was quarantined for several weeks, without railroad service or mail delivery.

At least 150 cases of "black small pox" were reported by February 1881, resulting in more than 82 deaths.

[9] According to the United States Census Bureau, the city has a total area of 0.50 square miles (1.29 km2), all land.

29.6% of all households were made up of individuals, and 12.9% had someone living alone who was 65 years of age or older.

25.9% of all households were made up of individuals, and 10.1% had someone living alone who was 65 years of age or older.