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 20,764 (31 August 2018) and covers an area of 1,823.90 square kilometres (704.21 sq mi) of which 252.57 km2 (97.52 sq mi) is water. There are all together around 465 lakes in Jämsä. The largest lakes are Päijänne, Koljonselkä and Lake Kuorevesi. The population density is 13.21 inhabitants per square kilometre (34.2/sq mi).
The municipality is Finnish.
The municipality of Kuorevesi was consolidated with Jämsä in 2001. The municipality of Längelmäki was partly consolidated with Jämsä in 2007. The municipality of Jämsänkoski was consolidated with Jämsä in the beginning of 2009.
It is the center of Finnish aeronautics industry – the aircraft factory and aeronautical engineering offices of Patria is located at Halli Airport in Jämsä.
Since 1994, the ski slope at Himos has been used as a super special stage at the Rally Finland.