Ostashkov is a town and the administrative center of Ostashkovsky District in Tver Oblast, Russia, on a peninsula at the southern shore of Lake Seliger, 199 kilometers (124 mi) west of Tver, the administrative center of the oblast. Population: 18,088 (2010 Russian census); 20,660 (2002 Census); 27,401 (1989 Soviet census).

Ostashkovsky District is an administrative and municipal district (raion), one of the thirty-six in Tver Oblast, Russia. It is located in the northwest of the oblast and borders with Demyansky District of Novgorod Oblast in the north, Firovsky District in the northwest, Kuvshinovsky District in the east, Selizharovsky District in the south, Penovsky District in the southwest, and with Maryovsky District of Novgorod Oblast in the west. The area of the district is 3,200 square kilometers (1,200 sq mi). Its administrative center is the town of Ostashkov. Population: 23,761 ; 7,357 (2002 Census); 8,921 (1989 Soviet census). The population of Ostashkov accounts for 76.1% of the district's total population.