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. 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 (2010 Census); 7,357 (2002 Census); 8,921 (1989 Census). The population of Ostashkov accounts for 76.1% of the district's total population.