Ionia is the largest city in, and the county seat of, Ionia County, Michigan, United States. The population was 11,394 at the 2010 census. Every late July (ending on the last Saturday of July and beginning 10 days previous) it hosts what is said to be the world's largest free-admission 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.
Ionia was historically served by multiple lines of the Pere Marquette Railway and the Grand Trunk Western Railway, but the only line still in service is the Mid-Michigan Railroad (former Grand Trunk Western) line to Grand Rapids.