Hagia Sophia (Ἁγία Σοφία), also known as "the Great Church" (Μεγάλη Ἐκκλησία), was the cathedral of Constantinople, erected in the center of the Constantinian city on the First Hill, very close to the Great Palace and the Hippodrome. Adjoined to the somewhat earlier church of Hagia Eirene, with which it shared the same enclosure, and together with the Hospice of Samson, they formed a large complex administered by the same clergy. The present building, erected by Emperor Justinian I in the sixth century, is the third church of Hagia Sophia at the site. The first Hagia Sophia, built in the second half of the 4th century and known simply as the Great Church, and the second 5th-century building were destroyed by fires in the course of riots in Constantinople. After being a museum for several decades, it began officially functioning as a mosque, known as Hagia Sophia Grand Mosque (Ayasofya-i Kebir Cami-i Şerifi), in 2020.

The plans of the first two churches were not recorded, but it is reasonable to assume that they were timber-roof, three-aisled or five-aisled basilicas. The first Hagia Sophia was consecrated in 360 in the reign of Constantine's son Constantius II. It burned together with the neighboring Senate during the 404 riot and was perhaps first repaired by Emperor Arcadius and then built anew or thoroughly reconstructed by Theodosius II (the inauguration of the church in 415). The only part that survived the fire was a circular building adjacent to the church; this may have been the same as the surviving skeuophylakion near the northeast corner of Hagia Sophia. The bread and wine for the liturgy in Hagia Sophia were prepared in the skeuophilakion, and gold and silver chalices, patens, gospel books and liturgical vessels were also stored there. In addition, written sources also enumerate a considerable number of precious relics that were kept in this building. The liturgical vessels and the relics were probably stored in rectangular round-headed niches that articulate the interior of the building on two levels (the upper niches accessible from a corbelled gallery to which an external staircase led). Other remains of the pre-Justinianic church were found in front of the narthex: The colonnade with a mosaic floor excavated before World War II belonged to the monumental gabled propyleum that preceded the atrium of the Theodosian cathedral.

The damage to the Theodosian cathedral during the Nika Riot in 532 was the great opportunity for Justinian to build a church that surpassed all ecclesiastical structures not only in the city, but also in the whole Roman and post-Roman world. Anthemios of Tralles and Isidore of Miletus were employed as master builders, and it is clear that they approached the design of the church from a theoretical perspective, disregarding the constraints of empirical building practice. The construction progressed rapidly and the new church, a colossal domed basilica with an apse projecting to the east and with a total length of around 135 m, was inaugurated on December 27, 537. The daring construction of the first dome and the fact that the church was erected with haste led to the collapse of the dome after the earthquakes of 557. Unfortunately, Procopius does not describe the the original dome in detail. On the other hand, it is clear from Agathias´ account of the rebuilding of the church that the original dome was larger and lower than the second dome, which was constructed by Isidore the Younger, nephew of the original engineer. The church was re-consecrated in 562. The dome of Hagia Sophia is slightly smaller than the dome of the Pantheon in Rome; it has a diameter of 31.87 m and a height from the floor level of 55.60 m. 

Hagia Sophia displays a radically innovative design, combining elements that had already emerged in Early Byzantine religious architecture (in the church of Sts Sergios and Bakchos, and maybe even in the church of St. Polyeuktos) but in an original way and on a vastly larger scale. The weight of the dome passes through the pendentives to four massive piers. The dome seems to float upon four great arches, of which the western and eastern ones are extended by half domes carried on smaller semidomed exedras. The central domed core of the church is flanked by aisles on the south and north sides and by two narthexes on the west side. Galleries above the aisles and the inner narthex were originally accessible by four spiral ramps adjoining the four corners of the church – now only that on the northwest corner is in use for this purpose.
The palace of the Patriarchs was also reconstructed after the Nika Riot. It was situated at the southwest corner of Hagia Sophia and comprised a long, possibly two-storied, vaulted hall flanked by smaller rooms. Only the north wall (with three large round-headed, now blocked, openings) and part of east wall, as well as springings of vaults remain of the large hall. These remains of the main reception space of the palace can be seen above the Ottoman ablution fountains. There are other rooms probably once belonging to the Patriarchate. These are well-preserved and are situated above the southwest vestibule of the church and on the top of the southwest access ramp. They are usually dated to the reign of Justin II and are decorated with figurative mosaics executed probably in the 9th century. Although directly accessible from the gallery of Hagia Sophia, they are not open to tourists.
The octagonal domed building (now one of the sultans´ mausolea), situated to the left when leaving Hagia Sophia through the southwest vestibul, may also have been originally part of the patriarchal palace. Its architecture and masonry point to the 6th century, which is usually identified as a baptistery. Although it served this function probably as early as the 9th- or 10th century, it may have been originally built as a small reception hall of the patriarchs or a chapel because Paul the Silentiary (later 6th century) in his description of Hagia Sophia mentions a baptistery that was apparently north of the church, which is also confirmed by other post-sixth-century sources. The Patriarchate was enlarged in the 7th century when the Thomaites Hall was added by one of the patriarchs of this name (Thomas I, 607-610, or Thomas II, 667-669).
Despite several additions and modifications made during its almost 1500-years-long history Justinian´s Hagia Sophia retains much of its original appearance. The outer buttresses are to be dated in the middle and late Byzantine periods: The flying buttresses on the west façade and those in the middle of both long sides as well as the south-west buttress belong to the former group while more massive buttresses on the east side of the church to the latter group (built by Andronikos II Palaeologos).
The silhouette of the church in 537 also differed from the present state as the dome was originally lower and its curvature was similar to that of the four pendentives marking the transition between the four big piers and the drum of the dome. The present dome rises about 6 m higher than the original one. Moreover, the interior of the church was brighter than it is today. Besides blocking some windows by adding the outer buttresses, the principal alteration of fenestration occurred after the first dome collapsed and was rebuilt: on the walls beneath the south and north big arches supporting the dome, were in 537 larger windows than the present ones (these are even smaller than after the rebuilding during Justinian´s reign due to later, probably Ottoman, interventions).15 In addition, the south-west and northwest porches do not seem to be part of the original design – the former was created when the rooms opening off the south gallery and belonging to the Patriarchate were built (which also included modification of the top of the south-west access ramp tower).
The most important loss is the disappearance of the Justinianic atrium that preceded the church on the west. The atrium extended westwards from the outer narthex for about 42 m and its three sides opened into the courtyard through pairs of columns alternating with piers. In the center of the atrium was an elaborate marble fountain. The atrium disappeared in the Ottoman period and the last columns and piers were demolished in 1870s. In addition Hagia Sophia was in the 6th century surrounded by courtyards on all sides;18 only the patriarchate, the Great Baptistery (on the north flank) and the earlier skeuophylakion adjoined the cathedral. The facades of Hagia Sophia were neither plastered nor presented their brick masonry, but they were faced with white Proconensian marble slabs.