Lumijoki is a municipality of Finland. It is located in the Northern Ostrobothnia region. The municipality has a population of 2,018 (31 December 2025) and covers an area of 290.29 square kilometres (112.08 sq mi) of which 77.1 km2 (29.8 sq mi) is water. The population density is 9.42 inhabitants per square kilometre (24.4/sq mi).