Jämsänkoski is a former town and municipality of Finland in the Central Finland region. It is located near Lake Päijänne and the Jämsänjoki river. The town had a population of 7,351 in 2008. It covered an area of 448.67 km² of which 48.02 km² is water. The population density was 16.9 inhabitants per km². The municipality of Koskenpää was consolidated with Jämsänkoski in 1969, while Jämsänkoski was consolidated with Jämsä in 2009.