Ilyas Bey Mosque, which is surrounded with a courtyard between the ruins of Miletus, was built in 1403 by Ilyas Bey (1402-1421) of the Turkish Mentse Emirate (Menteseogullari), who ruled in southwestern Anatolia for over a century until the Ottoman annexation of 1424. The building was repaired in 1905 for the first time when the dome was covered with brick tiles. It was restored from 1955 to 1972 by the General Directorate of Religious Endowments.
Ilyas Bey Mosque is a part of a complex, with the ruins of madrasa units on three sides of the mosque, and the tomb of Ilyas Bey to its north, all enclosed within a walled precinct. The mosque has a square plan, whose sides differ only half a meter, with an approximate length of eighteen and a half meters per sides. The portal on the north façade, which projects outwards, consists of a triple archway with stalactite capitals set in a large arched recess. Accessed by three steps the triple archway is framed by marble molding on the sides and topped by three blind arches without keystones, with polychrome tympana decoration.
The tympanum of the central arch contains three lines of carved Arabic inscriptions that precise the patron and give the date of completion of the mosque, 806 AH. While the wooden screens of the two side arches have remained, the wooden door of the central arch has not survived.