Jääsjärvi is a large lake in Finland. It covers 81.11 square kilometres (31.32 sq mi). It has many islands. The largest ones are Vehkasalo, Hirtesalo, Kotisalo, Ohrasaari, Urrionsaari and Nautsalo.
Municipalities near Jääsjärvi are Joutsa and Hartola. Jääsjärvi flows from the southwestern part via Tainionvirta and into several smaller lakes to Päijänne.