Sancaktar Hayrettin Mosque (Turkish: Sancaktar Hayrettin Camii; 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.

Location
The medieval structure, choked by artisan shops, lies in Istanbul, in the district of Fatih, in the neighbourhood of Kocamustafapaşa, on Teberdar Sokak, about five hundred meters north east of the Kocamustafapaşa station of the suburban railway line between Sirkeci and Halkalı.