Plain City is a village in Madison and Union counties in the U.S. state of Ohio, along Big Darby Creek. The population was 4,065 at the 2020 census. Until about 1800, the Ohio Country was inhabited by Mingo and Wyandot Indians, and there was an Indian village just north of present-day Plain City. After 1795, as white settlers began moving into the region, the area around present-day Plain City was referred to as Pleasant Valley. This name remained in use into the 20th century, appearing in the Ohio Guide of the late Depression era.