Alajärvi is a city in Southern Ostrobothnia, Finland. As of January 2014, about 10,250 people lived there. Nearby municipalities are Alavus, Kuortane, Kyyjärvi, Lappajärvi, Lapua, Soini, Vimpeli and Ähtäri. Alajärvi means also a lake, lake Alajärvi.
The former municipality of Lehtimäki was merged with Alajärvi on 1 January 2009.