Calhoun County is a county in the U.S. state of Illinois. As of the 2020 census, the population was 4,437, making it Illinois’ third-least populous county. Its county seat and biggest community is Hardin, with a population of 801. Its smallest incorporated community is Hamburg, with a population of 99. Calhoun County is at the tip of the peninsula formed by the courses of the Mississippi and Illinois rivers above their confluence and is almost completely surrounded by water.