St. Joseph County is a county located in the U.S. state of Michigan, on the central southern border with Indiana. As of the 2020 United States Census, the population was 60,939. The county seat is Centreville. French colonists in the late 17th century were the first Europeans to explore this territory, and they named the St. Joseph River for the patron saint of New France. This area was not part of the United States until after the American Revolutionary War.