The Rotunda of Agios Georgios or (in English) the Rotunda of St. George is one of the oldest and most imposing monuments in Thessaloniki.
It was part of a large complex that included the palace, an octagonal building and the Hippodrome, built by Caesar Galerius in the first Tetrarchy (around 300 AD), when he established Thessaloniki as his base.
The cylindrical structure was built in the 4th century AD on the orders of Galerius, who was thought to have intended it to be his mausoleum. It never served for this purpose, since Galerius died and was buried far away from Thessaloniki.
The temple was then converted into a Christian church and was possibly used as a Martyria, a place where the relics of the saints are worshipped. This theory is supported by both the circular form of the building and the images of saints depicted on the mosaics throughout the dome.