The Sheikh Suleiman Mosque (Turkish Şeyh Süleyman Mescidi) originally was a Byzantine structure near the Pantocrator Monastery in Constantinople. 
Neither its identity nor its function is known. Even though it often argued that it is a Palaiologan structure, the masonry indicates that it originated from an earlier period. It has been suggested that it was a mausoleum from the Early Byzantine era. Later it might have been used as a library of the Pantocrator Monastery. 
It consists of a central hexagonal superstructure with pendentives rising on a square substructure. There are four semicircular niches on the corners of the square and is surmounted by a shallow dome. Each of the walls that constitute the octagon have pointed arches on the outside, pointing to a later Ottoman renovation.  The southern niche was walled up and transformed into a mihrab when it was converted into a mosque. 
The building was converted into a mosque by Sheikh Suleiman in the reign of Mehmed II. It was damaged by a fire in 1756, and later restored. It has recently been renovated and is currently closed to the public.