The Diocese of Belley–Ars is a Latin Church ecclesiastical jurisdiction or diocese of the Catholic Church in France. Erected in the 5th century, the diocese was renamed in 1988 from the former Diocese of Belley to the Diocese of Belley–Ars. Coextensive with the civil department of Ain, in the Region of Rhône-Alpes, the diocese is a suffragan see of the Archdiocese of Lyon. The cathedra of the bishop is at Belley Cathedral. The current bishop is Guy Claude Bagnard, appointed in 1987.

Ain is a French department in the Auvergne-Rhône-Alpes region, Eastern France. Named after the Ain river, it is bordered by the Saône and Rhône rivers. Ain is located on the country's eastern edge, on the Swiss border, where it neighbours the cantons of Geneva and Vaud. In 2019, it had a population of 652,432.