The Diocese of Belley (Bellicium) is coextensive with the civil department of Ain and a suffragan of the Archbishopric of Besançon.
Although suppressed at the time of the Napoleonic Concordat, the diocese of Belley was re-established in 1822 and took from the Archdiocese of Lyons the arrondissements of Belley, Bourg, Nantua and Trévoux, and from the Archdiocese of Chambéry the arrondissement of Gex.
Local tradition maintains that Belley was evangelized in the second century by the martyrs Marcellus and Valerian, companions of St.