Haapajärvi is a town and municipality of Finland. It is located in the province of Oulu and is part of the Northern Ostrobothnia region. The town has a population of 6,500 and covers an area of 789.11 square kilometres of which 23.44 km2 is water. The population density is 8.48 inhabitants per square kilometre. Neighbour municipalities are Haapavesi, Kärsämäki, Nivala, Pihtipudas, Pyhäjärvi, Reisjärvi and Sievi. The municipality is unilingually Finnish.