Known since the XIV century. In the charter of the Lithuanian prince Olgerd to the Patriarch of Constantinople Philothe, dated 1371, is mentioned Klichen - a frontier town of the Moscow principality, located on the same island on Seliger. In 1393, Klichen was captured and burned by the Novgorodians. According to legend, after that, the only resident of Klichin, fisherman Eustafy (Ostashko), survived, who moved to the neighboring peninsula south of Klichen - from him the city and got its name.
In 1772-1775 - the center of Ostashkovsky district of Novgorod province. In 1775, the city and the county were transferred to the Tver governorate (from 1776 - Tver province). When re-planning in the county-level cities in the XVIII century, it was Ostashkov's new planning that was taken for exemplary for other uyezd cities of the Russian Empire (as in 1763 the new regular layout of Tver was recognized as a reference for provincial cities). In 1929-1935 Ostashkov was part of the Western region, from 1935 to 1990 - the Kalinin region.