The relic of the chains that bound Saint Peter when he was imprisoned in Jerusalem, the episode called the Liberation of Saint Peter. The Empress Eudoxia (wife of Emperor Valentinian III), who received them as a gift from her mother, Aelia Eudocia, consort of Valentinian II, presented the chains to Pope Leo I. Aelia Eudocia had received these chains as a gift from Iuvenalis, bishop of Jerusalem.

According to legend, when Leo, while he compared them to the chains of St. Peter's final imprisonment in the Mamertine Prison in Rome, the two chains miraculously fused together. The chains are kept in a reliquary under the main altar in the basilica.