Jääsjärvi is a medium-sized lake of Finland. It is located in Hartola and Joutsa municipalities, in the regions Central Finland and Päijänne Tavastia. The water quality in the lake is excellent. The river Tainionvirta has its source in Jääsjärvi. Jääsjärvi flows through it and with several smaller lakes to Päijänne. Water in Jääsjärvi is bright.