The Cathedral of St. William the Hermit (Spanish: Catedral de San Guillermo el Ermitaño), also known as the San Fernando Cathedral, is the seat of the Roman Catholic Diocese of San Fernando de La Union, in the Philippines (Dioecesis Ferdinandopolitana ab Unione). The diocese, which comprise the civil province of La Union, was created on January 19, 1970 and canonically erected on April 11, 1970 with Saint William the Hermit as the titular saint. It is a suffragan of the Archdiocese of Lingayen-Dagupan. Prior to the creation of the diocese, the church was formerly under the Archdiocese of Nueva Segovia.