The mosque called Yakup Çelebi, while lacking an inscriptive plaque (kitabe) to specify its date of construction and its donor, is assumed to have been donated by Yakup Çelebi, the son of Murad I and brother of Bayezid I in late 14th century. Built originally as a dervish lodge (zaviye), it was used a soup kitchen (imaret) towards the end of the empire. During republican years, the building became a museum warehouse. After its contents were transferred to the newly established Iznik Museum, the building was restored and converted into a mosque in 1963.

The building is based on a reverse T-plan and consists of a five-bay portico at entrance leading to a central court flanked by rooms east and west, rising to a large to the south. In an earlier resource, it is also described as having a basement*. The portico, currently englazed, is carried on four piers and two columns crowned with Byzantine capitals. It is roofed with mirror vaults; the vault covering the central bay is raised on a rectangular drum. Entering the central hall, doors immediately to the right and left open into vaulted guestrooms. Both rooms are equipped with fireplaces (ocak) and have a single window looking south. A dome resting on depressed arches east and west and ornamented with triangular planes in the zone of transition, encloses the central hall. The collapsed lantern of the dome was not restored. In 1963, a mihrab was added in the domed eyvan to the south to allow use as a mosque. The construction of the building is three layers of brick to a single layer of stone, with terra-cotta tiles on the domes. The interior has been entirely plastered during restoration.

In the plaza before the mosque, a tomb consisting of a dome elevated on four pedestals protects the symbolic grave of Yakup Çelebi, whose real tomb is in the mausoleum of Murad I (Hüdavendigar) in Bursa.

*Iznik (1943)

Sources:

1943. Iznik:Tarih ve Müze Komitesi Yayini. Istanbul: Kenan Matbaasi.

1972. Türkiye'de Vakif Abideler ve Eski Eserler. Volume IV. Ankara: Vakiflar Genel Müdürlügü Yayinlari.

Ayverdi, Ekrem Hakki. 1966. Osmanli mi'marisinin ilk devri : Ertugrul, Osman, Orhan Gaaziler, Hüdavendigar ve Yildirim Bayezid 630-805 (1230-1402) : I. Istanbul: Baha Matbaasi.

Eyice, Semavi. 1988. Iznik: Tarihçesi ve Eski Eserleri (Nicaea: The History and the Monuments). Istanbul: Sanat Tarihi Arastirmalari Dergisi Yayinlari.

Yalman, Bedri. 1999. Iznik (Nicaea). Bursa: Governorship of Bursa, Provincial Directorate of Tourism.