Sullivan County is a county in the U.S. state of Indiana, and determined by the US Census Bureau to include the mean center of U.S. population in 1940. As of 2020, the population was 20,758. The county seat is Sullivan.

The Terre Haute Metropolitan Statistical Area, also known as the Wabash Valley, is the 227th largest Metropolitan Statistical Area (MSA) in the United States. Centering on the city of Terre Haute, Indiana, it was originally formed by the United States Census Bureau in 1950 and consisted of Vigo County. As surrounding counties saw an increase in their population densities and the number of their residents employed within Vigo County, they met Census criteria to be added to the MSA. Four Indiana counties are now a part of this MSA.