Marttila (Swedish: S:t Mårtens, i.e. Saint Martin's) is a municipality in the region of Lounais-Suomi.It is located in the province of Western Finland and is part of the Southwest Finland region. The municipality has a population of 2,012 (31 January 2019) and covers an area of 195.99 square kilometres (75.67 sq mi) of which 0.71 km2 (0.27 sq mi) is water. The population density is 10.3 inhabitants per square kilometre (27/sq mi).
The municipality is unilingually Finnish.