Ionia is the largest city in and the county seat of Ionia County, Michigan, United States. The population was 13,378 at the 2020 census, and was estimated at 12,265 in 2024. Every July it hosts what is said to be the world's largest free-admission fair, the Ionia Free Fair. The city is mostly within Ionia Township, with small portion extending into Easton Township and Berlin Township, but is politically autonomous of the townships. The city is named after Ionia, a historic region in Greece.