The Rotunda of St. George is located just north of the junction of the two main axes of the city where the Arch of Galerius can be found. As one of the oldest monuments in Thessaloniki, its sixteen centuries of its existence, as pagan monument, Christian church and Muslim house of worship, have left their traces on it. It is now one of the 15 Paleochristian and Byzantine Monuments of Thessaloniki that were included in the UNESCO World Heritage List in 1988.
Although it is certain that the monument was erected by Galerius about AD 300, its purpose is less clear. Some scholars have asserted that it was built as a mausoleum for its founder, while others have suggested that it was a temple dedicated either to the Kabeiroi or to Zeus. The latter view is the more likely, since Zeus was the patron god of two of the tetrarchs, Diocletian and Galerius. It is certain, moreover, that Galerius died and was buried far from Thessaloniki. 
The building stood in a spacious precinct that ended at east and west in an exedra. Its circular shape - unique in Greece - connects it with the Pantheon of Agrippa in Rome. The cylindrical wall of the structure was constructed of rubble masonry, strengthened at intervals by wide zones of brickwork. The height of the building from the floor to the key of the dome is 29.80 m. The dome and arches are built solely of brick. In the key of the dome was an opaion to admit air and light. 
The building has a heavy, unarticulated exterior, while the interior is relieved by a large number of openings. At its base, there are eight large rectangular recesses set in the thickness of the cylindrical wall, with the area of wall remaining between giving the impression of huge pillars. These recesses are covered by barrel vaults, and were probably fronted by tribela with two columns supporting an epistyle. Above the recesses were eight large windows, and higher still, at the base of the dome, eight lunettes. The massive pillars, too, were relieved by the creation in them of small niches. These take the form of aedicules, their facades being formed of colonnettes standing on consoles and supporting an arch or a triangular pediment; inside them were placed small statues. Of the two niches that have not been closed, one still has its consoles. The main entrance to the building was the one in the south niche, corresponding with the axis of the Processional Way. The two pillars flanking it have spiral staircases inside them leading up to the roof. 
Both the date at which the pagan monument was converted into a Christian church, and its new name, are matters of dispute. As far as the date is concerned, this ranges from the late 4th to the early 6th century, the greatest probability being in the reign of Theodosius (AD 379-395). As for the name, the predominant view is that the church was dedicated to the Asomatoi or Archangels, with the name being extended to the nearby gate in the east wall and the district mentioned by various Byzantine sources. The church appears to have been the Metropolitan Church of Thessaloniki from 1523/24 to 1590/91, after the Church of Hagia Sophia was converted into a mosque. 
A large number of modifications were made to the building to adapt it to the new needs of worship, and some of them had a major effect on the statics of the structure that were felt as recently as the earthquakes of 1978. The most important intervention involved the demolition of the cylindrical wall at the east recess to accommodate the sanctuary, a rectangular area with a semihexagonal apse at the east end. This modification adversely affected the statics of the building and the apse received major repairs and was reinforced by two exterior buttresses. The arched opened for the sanctuary at some stage caused the collapse of the section of the dome above it, along with its mosaic decoration. 
Another important addition led to the area of the church being almost doubled by the construction of a closed ambulatory, 8 m wide, around the Roman core. This ambulatory communicated with the central area through seven of the recesses, which were pierced for the purpose. The ambulatory was destroyed, probably by the great earthquakes that struck Thessaloniki at the beginning of the 7th century. A new entrance with a narthex was created at the west niche. A propylon was added to the south entrance with two chapels, a circular one to the east and an octagonal one to the west. The emphasis placed on the importance of the south entrance by these new structures indicates that the axis from the Rotunda to the Palace, along the Processional Way, was retained; in the view of some scholars, this suggests that the Rotunda was converted into an imperial church during the Early Christian period. 
The most brilliant remains of the Early Christian phase of the monument, however, are its outstandingly beautiful mosaics — the earliest mural mosaics in the Middle East —which were the work of an excellent local mosaic workshop. These adorned the vaults over the recesses, the arched openings of the lunettes, and the dome; the cylindrical wall was revetted up to the base of the dome with slabs of colored marble. Nothing of this marble revetment has survived. 
The mosaics preserved on the vaults of three of the recesses and four of the lunettes have a purely decorative character. The impressive features of them are the wealth of motifs, the naturalistic treatment, the variety of odors, and the brilliance of the gold and silver tesserae used for the background. 
On the vault of the south-east recess, broad bands and circles form octagons enclosing birds and fruit alternately, while the broad band bordering the mosaic consists of panels containing vases with flowers and fruitstands with fruit. The decoration on the vault of the south recess gives the impression of a carpet: on the silver ground, around the gold cross in the centre, are spread flowers, birds, stars and baskets of fruit. The strictly geometric decoration of the vault of the west recess, with its intersecting circles, is again enlivened by birds and baskets of fruit, which fill the squares and circles forming the broad band around the edge. 
The decoration of the arches over the lunettes has a similar variety of motifs. The softer colors of the tesserae here reveal the skill and aesthetic sensibilities of the anonymous mosaicists, who adapted their color scale to the abundant direct light afforded by the lunettes, in contrast with the vivid colors they selected for the mosaics of the vaults, which were lit only indirectly. The culmination of this brilliant decoration is the mosaic decoration of the dome, arranged in three zones, progressing semantically from the zone of the martyrs, to that of the apostles or angels, with the divine Theophany of Christ Triumphant as the crowning piece. The lowest zone is divided into eight panels with decorative bands of repeated acanthus leaves. The eastern panel was destroyed when this section of the dome collapsed, and it now has a painted imitation of a mosaic, executed by the Italian artist Rossi in 1889. 
The background of each panel is occupied by light, two-story buildings composed of a variety of architectural elements, richly studded with pearls and precious stones. The origins of these architectural compositions go back to the Greco-Roman traditions, and recall the facades of theater stage-buildings, or of the rock-cut tombs at Petra in Arabia. The luxurious screens, the elegant lattice-works, the ciboria above the altars on which are precious gospel books, the crosses, the lighted candles and suspended chandeliers, all give the impression of the sanctuary of a church — not an earthly church but the heavenly church, as is clear from the birds of paradise scattered here and there and the peacock's feathers decorating the semidomes some of the niches. 
In front of these buildings is a line of fifteen still preserved martyrs, with tranquil expressions, each in an attitude of supplication. The inscriptions accompanying them give their name, their capacity, and the month of their festival. They include military saints, bishops, prelates, doctors, courtier and a servant, all of them from the Middle East, who form an illustrated calendar. Their bodies cannot be seen beneath their wide garments, but their faces, superbly worked in tesserae of varying sizes and colors, are true portraits. 
All that remains of the second zone are pairs of feet, wearing sandals and standing on grass. The traces of white, distinctly moving, drapery, hint at figures dressed in white and moving vigorously, perhaps apostles or angels. 
Of the culminating composition in the uppermost zone, there are three surviving heads — there would originally have been four — and a phoenix, the mythical bird that symbolized immortality and eternity. In their hands the angels are holding an astonishingly beautiful triple 'glory' composed of stars, a wreath of leaves and fruit, and a colored rainbow. Inside the glory, a preliminary sketch executed in charcoal can still be made out on the brickwork of the dome, reminding us that the figure of Christ, now lost, was depicted standing, holding a staff surmounted by a cross, a symbol of his victory over death. This figure summarizes the celebratory, eschatological meaning of the entire decoration, which begins with the other-worldly, static litany of the martyrs, continues with the lively procession of apostles or angels, and culminates in the triumphal appearance of Christ 'in glory'. 
Towards the end of the 9th century, the semidome over the sanctuary apse was painted with a scene of the Ascension. The Ascension prefigures the return to earth of Christ at the Last Judgment, and its symbolism is added to that of the sanctuary as a heavenly sanctuary and the seat of the Father, to which the Son ascends.
The arrangement of the figures in two zones — the Virgin, angels and apostles in the lower and Christ ascending in a glory held by angels in the upper — recalls the similar arrangement of the theme on the dome. This fact, and the stylistic affinities between the wall-painting and the mosaic Ascension on the dome of Hagia Sophia, have led some scholars to connect the two scenes, and even to attribute them to the same artistic workshop. 
The church was converted into a mosque in 1590-91. Ottoman tombs lie in a burial precinct to the east of the sanctuary of the church. Other remains dating from the Ottoman phase of the monument include the minaret, built inside the destroyed ambulatory, the fountain to the west of the church, and the porches above the west and south entrances. During this period, the monument is referred to as the Old Metropolis, or Eski Metropol, while it is called Rotunda by some of the foreign travelers of the 18th-19th century. The name Hagios Georgios, by which it is now known, derives from the small church dedicated to Saint George opposite the west gate in the enclosure wall of the Rotunda. After the liberation of Thessaloniki in 1912, the monument was given over to Christian worship until 1914, when large-scale archaeological excavations were begun in it. In 1917 it was converted into a "Macedonian Museum".