Ionia is the largest city in, and county seat of, Ionia County, in the U.S. state of Michigan. As of the 2000 census, the city population was 10,569, with the and estimated population of 12,449 as of the Census Bureau's July 2006 estimate. Every July it hosts the Ionia Free Fair. Ionia apartments for rent comparison page shows most up to date rent information on Ionia apartments and houses. Rent information is obtained from multiple rental sources but we cannot guarantee their accuracy.