The “Monastery Mosque” (Turkish Manastır Mescidi) is an unidentified Byzantine structure located on the seventh hill close near the Gate of St. Romanos (Turkish Topkapı). Its date is uncertain, though its masonry suggests a Palaiologan date.

Its original dedication is unknown, though it has been speculated that it belonged to the Monastery of Saints Menodora, Nymphodora, and Metrodora. The current building consists of a tripartite bema on its eastern end and a vaulted narthex on its western end. It also has two carved capitals in the triple arcade between the narthex and naos. The original plan of the building is unclear, though it seems that it was either a vaulted basilica or had cross-in-square plan. When the building was invested in the 1960s, foundations in the naos, apparently once supporting columns, were discovered along with evidence of an open portico surrounding the building to the north, south, and west. The size of the building suggests that it was a chapel within a monastery rather than its katholikon.

During the reign of Mehmed II the Conqueror (1451-1481), it was converted into a mosque by Mustafa Çavuş (after whom the mosque is also known). It was significantly altered after being converted into a mosque. The central apse was pierced by a window, while the south apse was filled and converted into a mihrab. Its present wooden roof is also a post-Byzantine addition. In the past, it had a wooden minaret but it did not survive. Originally it was the mosque of a mahalle (neighborhood), until it was absorbed into another mahalle in 1934. It is now partially enclosed by a bus depot. The area was altered in the 1950s with the construction of Millet Street a short distance from its northern wall.