Siuntio is a municipality in the Uusimaa region of Southern Finland. Its neighboring municipalities are Ingå to the west, Kirkkonummi, to the east, Lohja to the north-west, and Vihti to the north. It is 46 kilometres west of Helsinki. As of 2021, the population was 6,181 with a population density of 25.62 inhabitants per square kilometre. The municipality covers an area of 266.12 square kilometres, of which 24.96 km2 is water.