Seymour is a city in Jackson and Redding Townships, Jackson County, Indiana, United States, located 62 miles south of Indianapolis, and 55 miles north of Louisville, Kentucky. Its population was 21,569 at the 2020 census.

The city is known as the "Crossroads of Southern Indiana" for its location at the intersection of two major north–south and east–west railroads, which cross each other in the downtown area (and are still operating). The north–south line (the Jeffersonville, Madison and Indianapolis Railroad) was built in the 1840s and connected Indianapolis to the Ohio River at Jeffersonville. The east-west line (the Ohio and Mississippi Railroad) was built in the 1850s, connecting Cincinnati with St. Louis. The first settlers arrived at the intersection of these railroads - Seymour - in the spring of 1853. Additionally, since the 1920s two major U.S. Highways have also crossed at Seymour: U.S. Route 31 (Mackinaw City, MI to Spanish Fort, AL) and U.S. Route 50 (Ocean City, MD to Sacramento, CA). The southern section of U.S. Route 31 has largely been replaced in importance by the adjacent Interstate 65 (I-65).

The companies Aisin USA (the US branch of the Fortune Global 500 Company) and Rose Acre Farms (the 2nd largest egg producer in the United States) are headquartered in Seymour, and Cummins (another Fortune Global 500 Company) operates a plant in the area. Walmart operates a large distribution center east of the city near the junction of I-65 and US-50. The city is home to the 2nd largest high school gymnasium in the United States by seating capacity (Lloyd E. Scott Gymnasium), and Freeman Field, a historically significant WWII airbase that is now a civilian airport.