The former Breton and French diocese of Tréguier existed in Lower Brittany from about the sixth century, or later, to the French Revolution. Its see was at Tréguier, in the modern department of Côtes-d'Armor.

The title continues in the contemporary diocese of Saint-Brieuc and Tréguier.

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St. Tudgual (Tugdual, Tudual), said to be the nephew of St. Brieuc (who had emigrated from Cardigan), was a bishop who came to Brittany from overseas (Scotland), and was appointed by his uncle Brieuc at the close of the fifth century as superior of the monastery of Tréguier, which Tudual had founded. The biography of St. Tudual, composed after the middle of the ninth century, relates that Tudual, wishing to confirm his authority by royal approval, travelled to the court of King Childebert I, who ordered him consecrated Bishop of Tréguier. Louis Duchesne, however, argued that it was King Nomenoe who, in the middle of the ninth century, had the monastery of Tréguier raised to the dignity of an episcopal see.

Numerous synods were held at Tréguier in the fourteenth and fifteenth centuries, and passed regulations for the discipline of the Breton churches.

• Catholic Church in France
• List of Catholic dioceses in France

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