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. Since 1150, its bishop has been the Dean of the College of Cardinals.[1] 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.