Next to the Cathedral and the Baptistery stands the ancient Bishop's Palace, which can be assumed to be the continuation through many centuries of the first domus episcopi, an important part of the episcopal complex together with the two sacred buildings.
The domus of the bishops, from the late Middle Ages, when the written documentation begins, appears to have been the one that has come up to date, which reveals the same historical stratifications of the other palaces, public and private, of the ancient city. On the outside, the vast building appears, after the restoration in 1976, as an important late sixteenth-century building whose phases and chronology are evident. On the corner towards via Bernardo Ricci, the tower, incorporated in the subsequent structures and cut off at the height of the roof, has a very high face in small ashlars, which dates it back to quite ancient times. Towards via Bernardo Ricci the frescoed façade has been restored, with the inscription attributing it to the bishop Napoleone Fieschi (1463). On the long facade towards the Baptistery various phases and openings have been highlighted which correspond to as many loggias, which probably housed commercial activities well known from the documentation relating to the large market, the forum callegariorum which extended behind the Baptistery and the apses of the Cathedral and the wine forum just below the bishop's palace. On the first floor level, a large “heraldic” fresco with the coats of arms of Pope Sixtus IV della Rovere, bishops and lords helps to date this phase (1477); the fresco is attributed to Giovanni Canavesio, who also appears to have been chaplain in the Cathedral in the same years. The vast facade was later unified by the rebuilding of the entire building by the bishops GB Cicada and Luca Fieschi, with the arrangement of the beautiful cross cornices in Finale stone on the windows, which bear the coats of arms of the bishops and dates and had been removed. at the beginning of the 19th century, but preserved in the palace, in pieces. At the center of the sixteenth-century facade, the portal in the same stone still remembers the bishops and dates the work further.
Inside the building, the same construction phases are retraced and in the rooms of the museum the materials have been exhibited following a chronological criterion that is partly linked to the architectural phase of the rooms. In the first, housed in a beautiful fourteenth-century structure with a column and umbrella vault, the exquisite early medieval pieces from both the excavation of the Cathedral and the Baptistery, sculptures and epigraphic fragments are exhibited. The museum complex is introduced by the representation of the entire Diocese, a canvas by Bernardo Reubado, a local painter very close to the family of Bishop Pier Francesco Costa, almost an appendix to the description of the diocese, the well-known "Giardinello" that the bishop himself had drawn up.
In the second room, pertaining to the fifteenth-century phase towards via Bernardo Ricci, ceramic materials from the excavation of the Cathedral and fifteenth-century paintings, in particular by the De Rossi and the compartments with the saints Eligio and Ampelio of the polyptych attributed to Luca Baudo already in the Cathedral. The third room called “delle verzure” has revealed a fifteenth-century decoration with a drapery part that suggests it was the bishop's bedroom. There are exhibited, with others, the very valuable pieces of local goldsmiths, the reliquary heads of San Calocero and San Verano, also dated by written documents, and the fifteenth-century table with the "Saints Eleuterio and Placido" (1457). The next room, obtained from the tower and which became a chapel in the fifteenth century, has important frescoes. This is followed by the large sixteenth-century hall, where both local and Roman paintings are exhibited, due to the patronage of the Costa family: in particular, the "San Giovannino in the desert", a contemporary copy of the original by Caravaggio now in Kansas City, from the ancient parish church by Conscente, the canvas by Guido Reni, "The martyrdom of Santa Caterina" also from the parish church of Conscente and the altarpiece by Giovanni Lanfranco with "The miracle of San Verano", from the Cathedral. Still in the part of the monumental wing of the building facing the Baptistery, the Hall of tapestries, an important collection of Brussels artefacts with scenes from the life of Moses, and "entrefenetres" by Audenarde, due to the bequest of the eighteenth-century bishop De Fornari, to whom followed by the reception room of the bishops and that of the ancient vestments.