Kärsämäki is a municipality of Finland.

It is located in the province of Oulu and is part of the Northern Ostrobothnia region. The municipality has a population of 2,684 (30 June 2015) and covers an area of 700.87 square kilometres (270.61 sq mi) of which 6.06 km (2.34 sq mi) is water. The population density is 3.86 inhabitants per square kilometre (10.0/sq mi). Neighbour municipalities are Haapajärvi, Haapavesi, Pyhäjärvi, Pyhäntä and Siikalatva.