The diocesan museum is located in the building next to the Cathedral, commissioned by Cardinal Angelo Rapaccioli, bishop of Terni, as location for the diocese seminary. A building which, as was often the case in the old town of Terni, was built by linking together some small pre-existing houses of medieval origin, some of whose structures such as niches and columns came to light during the most recent restructuring which was carried out to adapt the structure to hold the works of art collected in the Diocese religious locations. Paintings, altarpieces and sacred objects which came from monasteries, churches, oratories and convents spread throughout the diocesan territory which included the territories of Terni, Narni and Amelia.
Among them the Circumcisim by Lucio Agresti, a Madonna with Child and saints by the Flemish painter Marten Stellaert and works by numerous painters from Umbria and Lazio among whom Girolamo Troppa and Liborio Coccetti, as well as a Lazurus' Resurrection attributed to the Guercino school.
The building, according to Cardinal Rapaccioli's wishes, was completely independent from the rest of the bishopry and from the cathedral church except for a gallery, now closed , which joined the seminary to the church. A building that the Cardinal wanted of a certain artistic validity and which was embellished with a series of frescos, now lost.
The diocesan Museum and chapter of Terni ,inaugurated in 2005, it is dedicated to the memory of don Fabio Leonardis, director of the diocese office of cultural heritage, and passionate curator of the project. On an area of four hundred square metres, In five halls that are spread over paintings and sculptures dating from the 15th to the 18th centuries are hosted, harmonized in a rich collection. This includes a vast section dedicated to contemporary artists such as Oliviero Rainaldi, Bruno Ceccobelli, Paolo Borghi, Riccardo Cinalli, Dino Cunsolo, some of whom realised their works in various town churches.