Isojoki is a municipality of Finland. It is part of the South Ostrobothnia region. The city of Pori is located 83 kilometres south of Isojoki. The population of Isojoki is 1,845 and the municipality covers an area of 642.38 km2 of which 5.05 km2 is inland water. The population density is 2.87/km2. The municipality is unilingually Finnish and neighbour municipalities are Honkajoki, Karijoki, Kauhajoki, Kristinestad, Merikarvia and Siikainen.