The Little Hagia Sophia mosque (Turkish: Küçük Ayasofya Camii), formerly the Church of Saints Sergius and Bacchus (Ancient Greek: Ἐκκλησία τῶν Ἁγίων Σεργίου καὶ Βάκχου ἐν τοῖς Ὁρμίσδου, romanized: Ekklēsía tôn Hagíōn Sergíou kaì Bákchou en toîs Hormísdou), is a former Greek Orthodox church dedicated to Saints Sergius and Bacchus in Constantinople (modern Istanbul), built between 532 and 536, and converted into a mosque during the Ottoman Empire.

[2] It was recognized at the time by Procopius as an adornment to the entire city,[3] and a modern historian of the East Roman Empire has written that the church "by the originality of its architecture and the sumptuousness of its carved decoration, ranks in Constantinople second only to St Sophia itself".

[4] The building stands in Istanbul, in the district of Fatih and in the neighbourhood of Kumkapı, at a short distance from the Marmara Sea, near the ruins of the Great Palace and to the south of the Hippodrome.

It was believed that the building had been designed by the same architects, Isidorus of Miletus and Anthemius of Tralles, as a kind of "dress rehearsal" for that of the largest church of the Byzantine Empire.

[1] Even so, because Anicia Juliana's St Polyeuctus, built 519–522, was not domed with brick but instead covered three opposing pairs of exhedrae with a wooden roof typical of a basilica, the immediate architectural precedent of both SS.

[8] During the years 536 and 537, the Palace of Hormisdas became a Monophysite monastery, where followers of that sect, coming from the eastern regions of the Empire and escaping the persecutions against them, found protection by Empress Theodora.

Then (between 1506 and 1513) it was transformed into a mosque by Hüseyin Ağa, the Chief Black Eunuch, custodian of the Bab-ı-Saadet (literally The Gate of Felicity in Ottoman Turkish) in the Sultan's residence, the Topkapı Palace.

[10] Due to the increasing threats to the building's static integrity, it was added some years ago to the UNESCO watch list of endangered monuments.

The building, the central plan of which was consciously repeated in the basilica of San Vitale in Ravenna and served as a model for the famous Ottoman architect Mimar Sinan in the construction of the Rüstem Pasha Mosque, has the shape of an octagon inscribed in an irregular quadrilateral.

Inside the edifice there is a beautiful two-storey colonnade which runs along the north, west and south sides, and bears an elegant inscription in twelve Greek hexameters dedicated to the Emperor Justinian, his wife, Theodora, and Saint Sergius, the patron-saint of the soldiers of the Roman army.