The Suburbicarian Diocese of Velletri–Segni is one of the Latin suburbicarian dioceses, Catholic dioceses in Italy close to Rome with a special status and a cardinal bishop, the bishop of Velletri–Segni. Historically, the see of Velletri was combined with the see of Ostia from 1060 to 1914.
The cathedral in the city of Velletri is dedicated to Saint Clement.
The Roman Catholic Suburbicarian Diocese of Ostia is an ecclesiastical territory located within the Metropolitan City of Rome in Italy. It is one of the seven suburbicarian dioceses. The incumbent bishop is Cardinal Giovanni Battista Re. For centuries, its bishop has been the dean of the College of Cardinals. Its Cathedral is Basilica di Sant'Aurea.
From 1105 to 1914, the diocese was merged with the Suburbicarian Diocese of Velletri. In 1962, the Diocese of Ostia was brought under the direct administration of the Diocese of Rome. Its territory is restricted to the single parish of Santa Aurea.