Sterzh is a lake in the Ostashkovsky district of the Tver region of Russia, the third largest in the system of the Upper Volga lakes in the Valdai Upland, the first of a group of lakes through which passes the upper course of the Volga River. Sterzh means "a quiet lake".
The length is 12 km, the width is up to 1.5 km, the average depth is 5 m, the largest is 8 m. The area is about 18 km².
The shores of the lake, which is a typical Seligersky stretch, are relatively high, the bottom and banks are sand and pebbles.
Earlier the lake was navigable, along it passed the passenger line Peno - Shirkov Pogost - Kokovkino with a length of 60 km, served by the ship "Zarnitsa".