It is bordered to the south by the Metropolitan City of Rome Capital and to the south-east by the province of Rieti.

[5] The territory falls in and makes up the largest part of the historical region of Tuscia, with the name sometimes being used synonymously with that of the province.

[6] After a period as a free commune, in the early 13th century Viterbo became part of the Papal States.

[6] The Knights of Saint John were expelled from Rhodes in the 16th century and were temporarily granted refuge in Viterbo before they could travel to Malta.

It joined the Kingdom of Italy on 12 September 1870 and during World War II, Viterbo was heavily bombed.