Fatih Mosque is a converted Byzantine church-mosque set in the center of the Bithynian city of Trigleia. It dates to the early 9th century and is a significant early example of a cross-in-square church. Although its Byzantine name is uncertain, it is often called the Church of St. Stephen, because of its association with the monastery of Trigleia whose abbot, St. Stephen the Confessor, suffered persecution in the reign of Leo V (813-820). It was converted into a mosque in the sixteenth or seventeenth century.

Its naos has a square plan, with a dome just under 5 meters in diameter, raised on a tall drum above four columns. The crossarms are covered by barrel vaults and the corner compartments have oval domical vaults. While its diakonikon is missing, its tripartite east end was quite large, with its lateral walls projecting beyond the width of the naos. The bema also has an extra bay before the apse. There is a broad, barrel-vaulted narthex to the west, which is preceded by a colonnaded portico. 

Much of the sculpture, including the capitals of the naos and closure panels, is reused from the sixth century, although the capitals of the lateral arcades and some of the cornice patterns, for example, might date to the ninth century. The interior was originally decorated with mosaics, which was noted during the Greek occupation in 1920-22, when the building was briefly reconverted to a church. There are mosaics in the soffits of the south arcade and east windows. A restoration of 1995-96 opened the arcades on the north and south sides of the naos, during which time fragments of opus sectile were also uncovered.