Kustavi is a municipality of Finland, located in the province of Western Finland and part of the Southwest Finland region. It is the smallest municipality in southwest Finland by population, with 972 residents.
The municipality covers an area of 770.15 square kilometres (297.36 sq mi), of which 603.45 km2 (232.99 sq mi) is water. Its population density is 5.86 inhabitants per square kilometre (15.2/sq mi).