Jämsä is a town and municipality of Finland. It is located in the Central Finland region, about 58 kilometres (36 mi) southwest of Jyväskylä. The municipality has a population of 19,889 (31 December 2020), which make it the second largest town of the Central Finland after Jyväskylä. It covers an area of 1,823.90 square kilometres (704.21 sq mi) of which 252.57 km² (97.52 sq mi) is water. The population density is 12.66 inhabitants per square kilometre (32.8/sq mi).

The municipality is officially Finnish-speaking.

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.