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. Its Cathedral is Basilica di Sant'Aurea.

Ostia Antica is the 35th zona of Rome, Italy, four kilometers away from the coast. It is identified by the initials Z. XXXV and it is distinct from Ostia. Ostia Antica belongs to Municipio X.