This is one of the oldest buildings in the city and is sixteen centuries old. It was erected around 300 AD by Emperor Galerius as a temple either of Cabeiros or Zeus. The interior of the monument is striking and the huge space beneath the vast dome- which in the light pouring in from the many windows appears to be suspended in space- inspires a sense of awe in the visitor. In the time of the Byzantine Emperor Theodosius the Rotunda was converted into a Christian church. During that period it was decorated with marvelous mosaics. In 1590 it became a mosque. Its minaret is the only one to have survived in Thessaloniki.

The oldest monument in Thessaloniki, the Rotunda (also known as Agios Georgios) is a massive round building that was first a Roman mausoleum, then a Christian church, then a mosque. Its interior is decorated with Early Christian mosaics; outside is the city's only surviving minaret.The Rotunda was built by the Roman emperor Galerius (305-11) as part of a large palace complex in Thessaloniki. It was probably intended to be his mausoleum, but it was never used as such.

Many Christian legends about the persecuting emperor's last years developed in Thessaloniki, such as his repentance on his deathbed as he suffered from painful and disgusting sores (an affliction ascribed to many enemies of the Church in early writings). It was also said that his daughter was secretly a Christian, who was martyred as well.

The Rotunda of Galerius was converted into a Christian church in the late 4th century or mid-5th century. The date of conversion to a church has been difficult to determine with any certainty. There is no written documentation about the event, so dating has to be based on analysis of the style of the mosaics and the historical situation of Thessaloniki in these early centuries.

With regard to the latter, Thessaloniki was always an important city and it was given a further boost after Constantine developed its harbor. The city became even more important after the mid-5th century, when it became a provincial capital and the base of the main military general of the west. Thessaloniki flourished, and developed at a fast pace into the mid-6th century.

Many grand churches were built in this period (including Panagia Acheiropoietos, St. Demetrios and Hosios David), and so it seems a logical period for the Rotunda transformation and decoration as well. However, the classical style and Early Christian themes of the mosaics have led other scholars to date it to the late 4th century, perhaps under the patronage of Emperor Theodosius I (379-95). The latter is adopted by most guidebooks.

The Ottoman Turks ruled Thessaloniki from 1430, and in 1591, Agios Georgios was converted into a mosque. On the Islamic calendar it was the year 999, when the end of the world was expected by some. Fortunately, the mosaics that survived until then were not harmed further by this conversion; they were simply painted over.

After serving three religions, the Rotunda is now a deconsecrated museum. It has been undergoing extensive restorations ever since the destructive earthquake of 1978. The Rotunda reopened in 1999.

The Rotunda is an austere structure built entirely of brick on a circle-within-a-circle plan. This circular building has huge dimensions. The strong walls are 6.30 metres thick. The interior diameter is 24.5 metres and its height to the top of the dome is 29.8 metres.To transform it into a church, Thessaloniki's Christians converted the southeast niche into an arch, added a sanctuary, moved the main entrance from the southwest to the northwest side, added a narthex, and built another ambulatory around the outside (which has not survived). The slender, freestanding Turkish minaret is the only one to have survived in Thessaloniki.

The interior was covered in glittering Byzantine mosaics, of which only a few fragments survive. Most of the damage was done by earthquakes over the centuries. They mosaics are Hellenistic in style and are some of the finest examples of mosaic work from this period. The quality of the work and complexity of the job suggests to some scholars that the workers were from Constantinople, probably sent by the emperor.

From the fragments and other sources it has been possible to reconstruct most of the original mosaic program, the overall theme of which seems to suggest the Second Coming of Christ.

The dome mosaic is now lost entirely, but its subject is known from the fortunate survival of its underdrawing. It depicted Christ, dressed in imperial purple and seated or standing on a shield. He held a cross in his left hand and raised his right hand upwards. This is a notable example of the influence of imperial elements in Early Christian art - the Roman army would often proclaim an emperor by elevating him on a shield.

This central scene was surrounded by 24 stars and a garland border and supported by four Victories or angels, whose heads and wing tips exist today. Between the angels were a phoenix and a luminous cross. Lower on the dome was a set of 24 to 36 standing figures, perhaps apostles and angels, of which only some sandaled feet remain.

The drum of the dome bears fragments of eight scenes, each in an architectural framework full of symbolic decoration (crosses, lamps, books, doves, peacocks, etc.) and centering on two or three martyred saints in prayer. Each saint wears slightly different vestments and has individualized features and a lively expression.

Inscriptions provide the name, occupation and festival month of each saint. They are all eastern saints and many were martyred under Diocletian (r.284-305). They are as follows:

Unknown (inscription lost); Leo (soldier, June); Philemon (flute-player, March)

Onesiphoros (soldier, August); Porphyrios (August)

Kosmas and Damian (doctors, September)

Unknown (inscription lost); Romanos (priest); Eukarpios (soldier, December)

Unknown (inscription lost); Ananias (priest, January)

Basiliskos (soldier, April); Priskos (soldier, October)

Philipos (bishop, October); Therinos (soldier, July), Kyrillos (bishop, July)

Lost panel

The choice and grouping of these saints is intriguing. There is no other known example in Byzantine art of this grouping of saints or even its compositional arrangement. Many of the festivals contradict known church calendars, both Greek and Latin, and some of the names (e.g. Basiliskos and Therinos) are not known anywhere else. There is no record in Byzantine writings of a soldier named Leo who was martyred in June.

Golden mosaics with medallions of birds and fruit (restored 1885) decorate the southeast recess, and the apse vault bears a faded fresco of the Ascension from the late 9th century.

In the yard around the Rotunda are marble fragments from the Byzantine church (including an ambo and various other carved marble slabs), Jewish tombstones, and other artifacts. Today it has the name of St George. It is called like because of the nearby chapel of that name while Rotunda was the name given in recent centuries by foreign visitors as a result of the circular shape of the monument.