Siuntio (Finnish: [ˈsiu̯ntio]; Swedish: Sjundeå) 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 (29 mi) west of Helsinki.
As of 2021, the population was 6,251 with a population density of 25.91 inhabitants per square kilometre (67.1/sq mi). The municipality covers an area of 266.12 square kilometres (102.75 sq mi), of which 24.96 km (9.64 sq mi) is water.