Balaban Ağa Mosque was a Byzantine structure in Constantinople that was converted into a mosque in the Ottoman Era. It was severely damaged by a fire in 1911 and demolished in 1930 to make way for Harikzedeler Street in Laleli. Before it was completely demolished, the Byzantine Institute and the Istanbul Archaeological Museums were able to excavate its substructure. 

While it no longer exists, written and visual records have survived. It had a hexagonal interior and a circular exterior with a diameter of 10.50 meters. Each side of the hexagon contained a rectangular niche about 2.7 meters wide. These niches were the same size, except for the western one, which was slightly larger and which opened on to the exterior through a door. The other niches contained windows, which might not have been original. The projecting piers between the niches preserved the comers of the hexagon as re-entrants. These piers rose to a height of about 2.15 meters, and caged six arches supporting a vault. At the springing of the arches was a single course of stone that penetrated the piers from site front to the back of the niches, but which did not pass right through the shell of the structure. Other than this single string course, the superstructure was almost entirely of brick.

The foundations excavated in 1930 were up 3 meters deep and built of both large and small blocks of stone. They were wider than the walls above them, and contained six irregular niches, which broadly corresponded to those of the building above. A burial was found in each of them, which suggests that the structure was originally designed as a mausoleum. The foundation walls of three of the niches possessed openings that must originally have connected with a subterranean channel, thereby ensuring that the crypt was well ventilated and dry.

In the 14th century, a rectangular chamber covered by a dome with a central oculus was sunk into the middle of the substructure. The large number of bones found within the chamber suggests that it was an ossuary in which the bones from the niches had been deposited so that new burials could be made in those niches. A slab covering one of the new graves was inscribed with the date 1345. 

The date and identity of the mausoleum is uncertain. However it has been compared to other mausoleums from Late Antiquity, such as the Mausoleum of Helena or Santa Costanza in Rome from the Constantinian Era. Further comparison has been made with other structures in Constantinople, including some of mausolea that were added to the Palace of Antiochus after its conversion into the Church of St. Euphemia or even with Şeyh Süleyman Mosque and the hypogeum in Hebdomon (modern Bakırköy). However these comparisons do not allow for a firm dating, since they date from the 4th to the 8th centuries. It has been proposed that the mausoleum might be linked to the 5th century Theotokos of Kourator, which was likely in the area. 
It was converted into a mosque by the Sekbanbaşı Balaban Ağa in 1483. In the course of conversion windows were pierced, a mihrab was built into the interior, and a portico was added to run along one-half of the exterior. A minaret was built on one of the piers.