It was then called Museo Capitolare as the museum pieces came from the Capitular Treasury of Velletri’s Cathedral.

The collection of religious art contains paintings, sacred vestments, jewellery and religious objects, among which is the famous Veliterna Cross, a reliquary cross of the 11th-12th century; la Visitazione (The Visitation) by Bicci Lorenzo (1435), La Madonna in Trono col Bambino (Madonna enthroned with Child) by Antoniazzo Romano (1486), la Madonna col Bambino ed Angeli (Madonna with Child and Angels) (1426-27) by Gentile da Fabriano and some ornamental parts of the late 12th century chasuble belonging to Pope Benedict XI.

The Diocesan Museum together with the Civic Museums is part of Velletri’s Museum Network.