Ostashkov is a town located in the Tver Oblast of Russia. It serves as the administrative center of the Ostashkovsky District and is situated on a peninsula along the southern shore of Lake Seliger. The town is positioned approximately 199 kilometers (124 mi) west of Tver, which is the administrative center of the oblast.
According to census data, the population of Ostashkov has fluctuated over recent decades. The population was recorded at 27,401 during the 1989 Soviet census, decreased to 20,660 in 2002, and stood at 18,088 as of the 2010 census.