Kasım Ağa Mosque (Turkish: Kasım Ağa Mescidi; also Kâsım Bey Mescidi, where mescit is the Turkish word for a small mosque) is a former Byzantine building converted into a mosque by the Ottomans in Istanbul, Turkey. Neither surveying during the last restoration nor medieval sources have made it possible to find a satisfactory answer as to its origin and possible dedication. It is probable that the small building was part of the Byzantine complex and monastery whose main church was the building known in Ottoman era as the Odalar Mosque, whose dedication is also uncertain. The edifice is a minor example of Byzantine architecture in Constantinople.