The Diocese of Saint-Brieuc and Tréguier (Latin: Dioecesis Briocensis et Trecorensis; French: Diocèse de Saint-Brieuc et Tréguier; Breton: Eskopti Sant-Brieg ha Landreger) is a Latin Church diocese of the Catholic Church in France.

A Welsh[1] saint, Brioc(us) (Brieuc), who died at the beginning of the sixth century founded in honour of St. Stephen a monastery which afterwards bore his name, and from which sprang the town of Saint-Brieuc.

[2] An inscription later than the ninth century on his tomb at Saint-Serge at Angers, where his alleged body was transported in the 850s,[3] mentions him as the first Bishop of Saint-Brieuc.

His alleged remains at Saint-Serge were moved to a different tomb in 1166, in the presence of King Henry II of England.

[6] Among the bishops of Saint-Brieuc, the following are mentioned: St. Guillaume Pinchon (1220–34), who protected the rights of the episcopate against Pierre Mauclerc, Duke of Brittany, and was forced to go into exile for some time at Poitiers; Jean du Tillet (1553–64), later Bishop of Meaux; Denis de La Barde[7] (1641–75); and Jean-Baptiste de Caffarelli du Falga (1802-15).

The principal pilgrimages in the Diocese of Saint-Brieuc are: Notre-Dame de Bon Secours at Guingamp, the sanctuary of which was enriched by the munificence of the Dukes of Brittany; Notre Dame d'Espérance, at Saint-Brieuc, a pilgrimage dating from 1848; Notre Dame de La Fontaine at Saint-Brieuc, dating from the establishment of an oratory by Saint-Brieuc, and revived in 1893 to encourage devotion to that Saint; Notre Dame de Guyaudet, near St-Nicholas du Pélem; and Notre Dame de LaRonce, at Rostrenen, a church raised to the status of a Collegiate Church by Sixtus IV in 1483.

An election was being prepared to choose his successor, when First Consul Napoleon Bonaparte ordered all the Constitutional Bishops to resign.

He was striking a Concordat with Pope Pius VII, which included the liquidation of the Constitutional Church.