Jefferson is a city in Union County, South Dakota, United States. The population was 475 at the 2020 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 "Jefferson Days", a festival that includes a movie in the park, a car show, fireworks and parade. The city has the name of President Thomas Jefferson.