Ilmajoki is a town and municipality in Finland, situated in the South Ostrobothnia region. It was founded in 1865 and is unilingually Finnish.
The municipality covers an area of 579.79 km², of which 2.89 km² is water, and has a population of 12,165. Its population density is 20.2 inhabitants per square kilometre. Ilmajoki borders the municipalities of Isokyrö, Kurikka, Laihia and Seinäjoki.