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 mean center of the United States population is determined by the United States Census Bureau from the results of each national census. The Bureau defines it as follows:The concept of the center of population as used by the U.S. Census Bureau is that of a balance point. The center of population is the point at which an imaginary, weightless, rigid, and flat surface representation of the 50 states and the District of Columbia would balance if weights of identical size were placed on it so that each weight represented the location of one person. More specifically, this calculation is called the mean center of population.