The Diocesan Museum of Velletri is located in the seventeenth-century ex-seminary of the complex annexed to the cathedral of San Clemente. The museum's collection includes works of art from the city's churches. The most valuable work preserved in the museum is the Crux veliterna, a reliquary in filigree gold, precious stones and enamels, a work of goldsmiths from Palermo dating back to the eleventh or twelfth century. According to tradition, containing a fragment of the Cross of Christ, it was donated by Frederick II of Swabia to Rinaldo of the lords of Ienne, then bishop of Velletri and later elected pope with the name of Alexander IV, who in turn donated it to the cathedral of Velletri. Among the other artistic works preserved in the museum, a Madonna with Child and two angels by Gentile da Fabriano, the only work of his stay in Rome between 1426 and 1427. In addition, there is a Madonna del Rosario by the Neapolitan school painter Sebastiano Conca . At the museum headquarters there is also an exhibition hall which hosts temporary exhibitions.