Marttila is a municipality of Finland, located in the province of Western Finland and part of the Southwest Finland region. It lies in the country's south-west, in the inland area of the region rather than on the coast.
The municipality has a population of 1,890 and covers an area of 195.99 square kilometres (75.67 sq mi), of which 0.71 square kilometres (0.27 sq mi) is water. With that population spread over that area, the population density is 9.68 inhabitants per square kilometre (25.1/sq mi), reflecting the sparse settlement typical of much of rural Finland.
Marttila is one of the smaller municipalities in the Southwest Finland region, and its territory is overwhelmingly land, with only a small fraction of its surface area accounted for by water.