Ostashkov is a town and the administrative center of Ostashkovsky District in Tver Oblast, Russia, located on a peninsula at the southern shore of Lake Seliger, west of Tver, the administrative center of the oblast. Population:

The island of Klichen was first mentioned in a letter sent by Grand Duke Algirdas of Lithuania to the Ecumenical Patriarch of Constantinople in 1371. After the island was pillaged by Novgorod pirates several years later, two of Klichen's surviving inhabitants, Ostashko and Timofey, moved to the mainland, where they founded the villages Ostashkovo and Timofeyevo, respectively. The former belonged to the Moscow Patriarchs, and the latter—to the Joseph-Volokolamsk Monastery. In 1770, both villages were merged into the town of Ostashkov.