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 (29 mi) west of Helsinki.

Uusimaa is a region of Finland. It borders the regions of Southwest Finland, Tavastia Proper (Kanta-Häme), Päijänne Tavastia (Päijät-Häme), and Kymenlaakso. Finland's capital and largest city, Helsinki, along with the surrounding metropolitan area, are both contained in the region, and Uusimaa is Finland's most populous region. The population of Uusimaa is 1,734,000.