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.