Posey County is the southernmost, southwesternmost, and westernmost county in the U.S. state of Indiana. Its southern border is formed by the Ohio River, and its western border by the Wabash River, a tributary to the Ohio. As of 2020, the population was 25,222. The county seat is Mount Vernon. Posey County is part of the Evansville, Indiana metropolitan statistical area. The Ports of Indiana-Mount Vernon, on the Ohio River, is the seventh-largest inland port complex in the nation.