Preble County is a county located in the U.S. state of Ohio. As of the 2020 census, the population was 40,999, down 3.0% from the 2010 census population of 42,270. Its county seat is Eaton. The county was formed on February 15, 1808, from portions of Butler and Montgomery Counties. It is named for Edward Preble, a naval officer who fought in the American Revolutionary War and against the Barbary Pirates.