Jefferson is a city in Union County, South Dakota, United States. The population was 547 at the 2010 census. It is part of the Sioux City, IA–NE–SD Metropolitan Statistical Area. Jefferson was founded in 1859. Every year the citizens of Jefferson and surrounding area celebrate the founding of Jefferson during the "Days of '59", a festival that includes softball, basketball, horseshoes, car show, fireworks and parade.