Saint Emygdius (Latin: Emidius, Æmedius, Emigdius, Hemigidius; Italian: Sant'Emidio; c. 279 – c. 309 AD) was a Christian bishop who is venerated as a martyr.

The people of Rome believed him to be the son of Apollo and carried him off by force to the Temple of Asclepius on the island in the Tiber, where he cured many of the sick.

Enraged, Polymius decapitated him on the spot now occupied by the Sant'Emidio Rosso temple, as well as his followers Eupolus (Euplus), Germanus, and Valentius (Valentinus).

Emygdius stood up, and carried his own head to a spot on a mountain where he had constructed an oratory (the site of the present-day Sant'Emidio alle Grotte).

[1] His hagiography was written probably by a monk of Frankish origin in the eleventh century, after the rediscovery of the saint's relics, which had been conserved in a Roman sarcophagus.

The translation of his relics from the catacomb of Sant'Emidio alle Grotte to the crypt of the cathedral happened probably around the year 1000 under Bernardo II, bishop of Ascoli Piceno.