The Monastery of Saints Sergius and Bacchus at Hormisdas (Μονὴ τῶν Άγίων Σεργίου καί Βάκχου ὲν τοῖς Ὸρμίσδου) was built during the reign of Justinian. It is located near the Sea Wall by the Marmara Sea south of the Hippodrome of Constantinople. It was built on the grounds of the Palace of Hormisdas next to the Basilica of Sts. Peter and Paul. The church was dedicated to the saints Sergius and Bacchus, Roman soldiers who were martyred under Emperor Maximian around 300. Its association with Hagia Sophia is reflected in its Turkish name, Küçük Ayasofya Camii (“Little Hagia Sophia Mosque”).

The Church of Ss. Sergius and Bacchus was built on the grounds of the Palace of Hormisdas, which was named after a Sassanid prince who was received at the court of Emperor Constantine after fleeing from Persia in 324. The palace was the residence of Justinian when his uncle Justin I (518-527) was the ruling emperor. After consecrated the Basilica of Ss. Peter and Paul at the palace in 518-519, he asked Pope Hormisdas for the relics of the two apostles in Rome. The precise date of Sergius and Bacchus is controversial. Traditionally it was been argued that it was a palatine chapel built by Justinian and Theodora built around 527 when the imperial couple moved to the Great Palace complex. However, it has also been argued that it was built by the year 536 for Syrian Monophysite monks who were being supported by Empress Theodora. It is known that the palace accommodated over 500 Monophysite refugees during this period. The church was first attested – as a monastery under Abbot Paul – in 536 when the anti-Monophysite council of 536 was held here. Following the death of Theodora in 548, its Monophysite inhabitants were moved to the house of Urbicius. Around 547, Pope Vigilius, fearing Justinian’s wrath during the Three Chapters Controversy (related to the Nestorian heresy), sought refuge at the monastery and attempts to remove him by force failed due to the protests of the populace. It also has stylistic affinities with the Church of St. Polyeuctus built by Anicia Juliana built a short time earlier.

This church remained a monastery throughout the Byzantine era. The church was a center of Iconoclasm under its hegoumenos (abbot) and future Patriarch John VII Grammatikos during the reigns of Leo V and Theophilus. As hegoumenos (c. 815-37) he interrogated many prominent supporters of icons, including Theodore of Stoudios and Theophanes the Confessor, at Sergius and Bacchus. Sergius and Bacchus was restored by Basil I after 867 and was granted to the See of Rome in 880 during the reign of Basil I, though it seems to have already been placed in the service of the Pope earlier. The future Patraich Euthymios rejected Leo VI’s attempt to make him the hegoumenos. Its fate during the Latin occupation of the city is unknown, but it is mentioned by pilgrims visiting the city following the Byzantine reconquest of the city in 1261. In the early 16th century, the church was converted into a mosque by Hüseyin Agha, the chief eunuch of the harem of Bayezid II. His türbe (tomb) is now in the middle of the cemetery north of the mosque, which is often named after him as well. The building was restored in 1740 and again after the fire of 1758. It housed refugees from the Balkan in the early 20th century and was restored again in 1956.

Serigus and Bacchus, along with its more famous and impressive counterpart Hagia Sophia, reveals the inventiveness of Justinian’s construction projects in Constantinople. It has often been considered an important precursor of Hagia Sophia, but as its date is controversial, this is far from certain. It shared the same atrium with the adjacent church of Ss. Peter and Paul that was probably a basilica. It has been argued that this accounts for its irregular plan. While it is not very large (measuring around 30 x 34 meters), it is designed in such a way that produces a sense of spaciousness inside. This structure can be compared with the churches of San Vitale in Ravenna and the non-extant St. John Prodromos at Hebdomon.

The brick-and-mortar building has an octagonal nave set within a irregular rectangle. It has a double-storied arcade that supporting a pumpkin dome (17 meters in diameter) with 16 alternating curved and flat wedges. The flat surfaces are pierced by windows, while the curved wedges coincide with the corners of the octagon. There is a broad arch over the apse on its east side and seven narrower arches over the other piers. The arches of the octagon frame two-story niches of alternating rectangular and semicircular plan, while the angles of the octagonal nave are marked by eight wedge-shaped piers. Paired columns of verde antico and red-veined Synnada marble support a richly carved horizontal entablature made of Proconnesian marble at the gallery level. These columns are capped with delicately undercut “melon” or “folded” capitals, which emphasize the crosses and imperial monograms in the center. The columns of the gallery also include Ionic impost capitals. Along its entire length is an inscribed epigram with expertly carved letters in honor of Justinian and Theodora. The entablature also includes the traditional egg-and-dart, bead-and-reel and acanthus rinceau. While it would have originally been decorated with mosaics and polychrome marble revetment on its walls and piers, it is now mostly covered with plaster. During the Ottoman era, additions include a portico at the entrance and a minaret, while the dome was sheathed in lead and the windows were altered. A tekke (sufi lodge) with thirty-six chambers was founded around the mosque, while a fountain was built in its courtyard.