The Diocese of Saint-Brieuc and Tréguier is a Roman Catholic diocese located in the region of Brittany, France. Established in the 5th century, it encompasses the departments of Côtes-d'Armor and parts of Ille-et-Vilaine. The diocese is named after its two main cities, Saint-Brieuc and Tréguier, which serve as important religious and cultural centers. The diocese is part of the ecclesiastical province of Rennes and is governed by a bishop. It plays a significant role in the spiritual life of the local community, overseeing various parishes and promoting religious education. The diocese also engages in social outreach and charitable activities throughout the region.