Schaalsee is a 24 km2 (9.3 sq mi) natural lake in Germany. It forms part of the border between Schleswig-Holstein (district Herzogtum Lauenburg) and Mecklenburg-Vorpommern (districts Ludwigslust-Parchim and Nordwestmecklenburg). The town of Zarrentin is located on its southern shores. Other municipalities on the lake are Seedorf, Sterley, Salem, Kittlitz and Kneese.
It was declared a biosphere reserve in 2000 (309 km2).
