Champaign County is a county located in the U.S. state of Ohio. As of the 2020 census, the population was 38,714. Its county seat and largest city is Urbana. The county takes its name from the French word for "open level country". Champaign County became the 18th of 88 Ohio counties on March 1, 1805. It was formed from parts of Greene and Franklin counties by legislative action.