early Christian bishop, martyr, and one of the patron saints (Saint Elmo's fire) of sailors, who is romantically associated with Saint Elmo's fire (the glow accompanying the brushlike discharges of atmospheric electricity that appears as a tip of light on the masts of ships during stormy weather) as the visible sign of his guardianship over them. Erasmus is one of the Fourteen Holy Helpers, a group of saints conjointly venerated in medieval Germany.

He is reported to have been bishop of Formia, where he was martyred, probably during the persecution of Christians by the Roman emperor Diocletian. According to Pope Gregory I (reigned 590–604), his relics were kept in the Cathedral of Formia. After the Saracens destroyed Formia in 842, Erasmus' body was transferred to Gaeta, Italy, where he is honoured as patron saint.

Several spurious acta have embellished his legend. According to these, he was a bishop in Syria who miraculously endured tortures under Diocletian in Lebanon, after which he was guided by an angel to Formia, where he performed many miracles. He has been confused with the Syrian St. Erasmus of Antioch; some scholars propose that they are the same person. Later legends attest that he was martyred by being disemboweled; thus, as a Holy Helper, he was invoked by those suffering from intestinal maladies. Elmo is an Italian corruption (through Sant' Ermo) of St. Erasmus; other derivations include Ramus, Eramus, Ermus, Ermo, and Telmo. His legendary narrative is in Acta Sanctorum.