Jefferson 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 586 at the 2000 census. Jefferson was founded in 1859. Every year the citizens of Jefferson and surrounding area celebrate the founding of Jefferson during the Days of '59 celebration, a weekend-long festival that includes softball, basketball, horseshoes, and a poker tournament, as well as a car show and parade.