The Arch and Rotunda of Galerius formed part of an imperial precinct created when Thessaloniki became a residence of the Roman tetrarch Galerius near the end of the third century. The city already commanded the Via Egnatia, the great road linking the Adriatic with the eastern empire. Galerius added a palace, hippodrome, monumental avenue, ceremonial arch, and domed rotunda, turning urban circulation into a display of imperial order. The surviving monuments are fragments of that planned ensemble rather than isolated commissions.

The arch was constructed around 298 to 303 to commemorate Galerius's victory over the Sasanian Persian king Narseh. Originally an eight-piered gateway with multiple openings, it stood where a processional route from the palace crossed the Via Egnatia. Marble reliefs show battles, submission, sacrifice, personified rivers, and the four tetrarchs acting in harmony. They are political images rather than documentary battle scenes: Galerius appears personally defeating Narseh, an encounter that did not occur. Only three western piers and portions of the arches survive, enough to convey the monument's scale and message.

North of the arch, the Rotunda was built around 306 as a massive circular hall beneath a brick dome. Its original purpose remains debated. It may have been intended as Galerius's mausoleum, an imperial audience chamber, a temple, or a building capable of several ceremonial uses. Galerius was buried elsewhere after his death in 311. The walls, more than six meters thick in places, allowed the dome to survive repeated earthquakes. An opening toward the processional avenue connected the building visually and politically to the palace complex.

By the late fourth or early fifth century the Rotunda had become a Christian church. An apse and ambulatory altered its plan, while glittering mosaics transformed the upper walls and dome. Surviving figures of saints stand before fantastic golden architecture below a lost central vision, joining Roman engineering to early Christian worship. The conversion reflects Thessaloniki's growing importance within the Christian empire. Later repairs changed openings and surfaces, but the building served a church community for more than a thousand years and became associated with the Archangels and eventually Saint George.

Ottoman forces captured Thessaloniki in 1430, and in 1590 the Rotunda was converted into a mosque. A minaret rose beside the ancient cylinder, adding a new layer without destroying every Christian mosaic. After Greek forces took the city in 1912, the building returned to Christian and archaeological use, while the minaret remained as evidence of its Ottoman history. A major earthquake in 1978 damaged the Rotunda and accelerated long conservation efforts. Urban traffic, pollution, moisture, and seismic risk continue to affect both it and the exposed arch.

The Rotunda joined UNESCO's Paleochristian and Byzantine Monuments of Thessaloniki designation in 1988, while the arch remains essential to understanding its Roman setting. Religious services still occur on selected days, but the Rotunda is principally managed as a monument. Together the two structures condense successive identities of Thessaloniki: imperial capital, Christian metropolis, Ottoman city, Greek national center, and modern archaeological landscape. Their significance comes from survival through transformation, not from an imagined original purity; reliefs, mosaics, minaret, repairs, and missing sections all testify to changing claims on urban space.