Jämsä is a town and municipality of Finland.

It is located in the province of Western Finland and is part of the Central Finland region. The municipality has a population of 21,758 (30 June 2015) and covers an area of 1,823.91 square kilometres (704.22 sq mi) of which 252.57 km (97.52 sq mi) is water. There are all together even 465 lakes in Jämsä. Largest lakes are Päijänne, Koljonselkä and Lake Kuorevesi. The population density is 13.85 inhabitants per square kilometre (35.9/sq mi).