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Ostashkov (Russian: Оста́шков) 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 Census); 20,660 (2002 Census); 27,401 (1989 Census).

Ostashkovsky District (Russian: Оста́шковский райо́н) 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.