Päijänne is the second largest lake in Finland. It is between the regions of Central Finland and Päijänne Tavastia. It covers 1,100 kilometres (680 mi). Päijänne flows into the Gulf of Finland through the river Kymijoki.
The quality of the water is good. It improves south of Jämsä.
The lake has 1,886 islands.
Some of the largest islands are Virmailansaari, Salonsaari, Judinsalo, Onkisalo, Paatsalo, Muuratsalo, Haukkasalo and Vuoritsalo.
In the southern part of Päijänne is Päijänne National Park. This was established in 1993. The park covers about 50 islands. The park is next to the municipalities of Padasjoki, Asikkala and Sysmä. It covers Kelvenne island, which is 8 km long and 50-800 meters wide. The bays in the national park are popular places for boating.