The diocese of Bagnoregio is a former Roman Catholic territory in Lazio, Italy.
According to tradition, St. Ansanus preached the Gospel here in the third century and the church of Santa Maria delle Carceri outside the Alban Gate was said to have been built above the prison in which he was confined. There are no records as to the date of the erection of the diocese but a letter of Pope St. Gregory the Great is authority for the statement that about the year 600 the newly elected deacon John was appointed bishop of Bagnoregio, the earliest extant mention of a bishop of the see, but he was doubtlessly not the first bishop.
The diocese grew over the centuries, gaining terrotories around 90 from the dioceses of Perugia and Orvieto and incorporating in 1015 what had been the Roman Catholic Diocese of Bomarzo.
Up to the time of pope Urban V, Montefiascone was part of the Diocese of Bagnorea (Bagnoregio's Latin name), but was made by this pontiff the seat of a new diocese. Ferdinando Ughelli, without any documentary proof, claims the Diocese of Bagnorea was joined to the Diocese of Viterbo on 4 February 1449, but neglects to mention when they were reestablished as separate dioceses.
After an earthquake in 1695, the cathedral that had been in Civita di Bagnoregio was replaced by one at Bagnoregio itself.
On September 30, 1986 it was one of several former bishoprics to be suppressed and merged into the neighbouring Diocese of Viterbo e Tuscania, that of Viterbo, by whose bishop it was already administered since the death of the last diocesan bishop of Bagnoregio in 1971. The merged bishopric then took the name of Diocese of Viterbo, Acquapendente, Bagnoregio, Montefiascone, Tuscania and San Martino al Monte Cimino, in 1991 again shortened to Bishopric of Viterbo.