Jämsänjoki is a river in Finland in the town of Jämsä in Central Finland region. It is 15 kilometres (9 mi) long and has a watershed of about 1,448 square kilometres (559 sq mi). The river flows from Kankarisvesi into the Lake Päijänne, from which the waters flow into the Gulf of Finland through the Kymijoki river.
Coordinates: 61°49′N 25°15′E / 61.817°N 25.250°E