It is the church of the patron saint of Thessaloniki. It was built above the ruins of the roman baths on the site where St Demetrius met a martyr’s death. Initially, in the fourth century, a small structure stood on the site where Demetrius was first imprisoned and then interred. In the fifth century, Leontius, Eparch of Illyricum, built a spacious three-aisled basilica which was burnt during the seventh century. It was rebuilt with five aisles, but that structure, too, met its end by fire in 1917. During the restoration of the monument after the fire of 1917, several trenches were opened in the naos and the crypt. Systematic excavations were carried out in 1946-49. Wonderful mosaics have survived in the basilica from the fifth, the seventh, and the ninth century together with some wall-paintings. The relics of the saint are kept inside a silver reliquary beneath a replica of the original ciborium.

In the crypt of Saint Demetrius, beneath the altar, we can visit the Roman baths in which the saint is traditionally supposed to have martyred. The holiest part of the crypt is the semi circular basin of holy water.The famous holy myrrh of the saint ran from there. The pilgrims used to bear away this myrrh in phials to the time of the Ottoman Rule, when the church became a mosque.

In the crypt of Aghios Demetrios, an exhibition is on display to the public. It includes the items that survived the fire of 1917 and those that were brought to light by the recent excavations in the monument.