The Diocese of Saint-Brieuc and Tréguier is a Latin Church diocese of the Catholic Church in France. The diocese comprises the department of Côtes d'Armor in the Region of Brittany. The diocese is currently suffragan to the Archdiocese of Rennes, Dol, and Saint-Malo. The current bishop is Denis Moutel, appointed in 2010.

The Côtes-d'Armor, formerly known as Côtes-du-Nord until 1990, is a department in the north of Brittany, in northwestern France. In 2019, it had a population of 600,582.