It was built by Galla Placidia, as votive for a missed shipwreck while she was traveling from Constantinople to Ravenna in 424 for the enthronement of her son Valentiniano III. The three nave church underwent various interventions during various epochs until it was finally nearly rebuilt totally during the air bombings of 1944. In front of the building there is a quadrangular enclosure with a marble portal of the 4th century on the lunette of which the apparition of St. John to Galla Placidia is represented.