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 . Its administrative center is the town of Ostashkov. Population: 23,761 (2010 Census); The population of Ostashkov accounts for 76.1% of the district's total population.