It is located in the province of Western Finland and is part of the Southwest Finlandregion. The municipality had a population of 1,321 (December 31, 2004) and covered an area of 176.53 km2 of which 16.38 km2 is water. The population density was 8.25 inhabitants per km2. Its administrative center is Kitula.
Suomusjärvi was first mentioned in 1540, when it was a part of the parish of Kisko. It got its own chapel community in 1678. The chapel community was also known as Laidike as the church of the community was located in the village of Laidike. A new church was built in the village of Suomusjärvi in 1703, but the parish was still called Laidike in 1722. Suomusjärvi became an independent parish in 1898.[1]