Sancaktar Hayrettin Mosque (Turkish: Sancaktar Hayrettin Câmîi; also Sancaktar Hayrettin Mescidi, where Mescit is the Turkish word for a small mosque, or Sancaktar Mescidi) is part of a former Eastern Orthodox monastery converted into a mosque by the Ottomans. It is generally believed that the small building belonged to the Byzantine Monastery of Gastria (Greek: Μονῆ τῶν Γαστρίων, Monē tōn Gastríōn, meaning "Monastery of the Vases"). The edifice is a minor example of Palaiologan architecture in Constantinople, and is important for historical reasons.