Known since the XIV century. In the letter to the Lithuanian Grand Duke Olgerd Philotheus the Patriarch of Constantinople, dated 1371 year, referred to Klichen - the border town of Muscovy, located on the same island on lake Seliger. In 1393 Klichen was captured and burned by Novgorod's troops. According to legend, survived only one resident of Klichen fisherman, which name was Eustace (Ostashko). He moved to the neighboring peninsula south of the Klichen. The town was named in honor of this fisherman.
In 1772-1775 - centre of Ostashkov district ща Novgorod guberniya. In 1775 the city and the district have been transferred to the governorship of Tver vicegerency (from 1776 - Tver guberniya). When remodeling a district-level cities in the XVIII century it was the new plan of Ostashkov was taken as a model for other other district-level cities of the Russian Empire (in 1763 a new regular planning of Tver has been recognized as the standard for guberniya cities). In 1929-1935 Ostashkov was a member of the Western region, from 1935 to 1990 - the Kalinin region.