Samar and Leyte, two civil provinces in the Visayan group of the Philippines, which include the islands of Balicuatro, Batac, Biliran, Capul, Daram, Homonhon, Leyte, Manicani, Panaon, Samar and several smaller islands, once made up the diocese of Calbayog, now a suffragan of the Metropolitan Archdiocese of Palo.

The Diocese of Calbayog is the local church comprising the civil territorial jurisdiction of western Samar Island.

The City of Calbayog is where the Cathedral of the diocese is located since its ecclesiastical foundation on April 10, 1910, by Pope Pius X.

On April 28, 1934, Pope Pius XI promulgated an apostolic constitution with the incipit Romanorum Pontificum semper separating the dioceses of Cebu, Calbayog, Jaro, Bacolod, Zamboanga and Cagayan de Oro from the ecclesiastical province of Manila.

The historical vicissitudes of the Diocese of Calbayog cannot be fully appreciated apart from the history of the early evangelical works of the first missionaries who came to Samar.