Alajärvi is a town and municipality of Finland. It is located in the South Ostrobothnia region. The town has a population of 9,113 and covers an area of 1,056.74 square kilometres (408.01 sq mi) of which 47.99 km2 (18.53 sq mi) is water. The population density is 9.03 inhabitants per square kilometre (23.4/sq mi). The town is unilingually Finnish; there are few immigrants.

South Ostrobothnia is one of the 19 regions of Finland. It borders the regions of Ostrobothnia, Central Ostrobothnia, Central Finland, Pirkanmaa, and Satakunta. Among the Finnish regions, South Ostrobothnia is the ninth largest in terms of population. Seinäjoki is the regional centre and by far the largest city in the area.