Tréguier Cathedral (Cathédrale Saint-Tugdual de Tréguier) is a Roman Catholic church and former cathedral in Tréguier in Brittany, dedicated to Saint Tudwal.
The biography of St. Tudgual, composed after the middle of the ninth century, relates that King Childebert had him consecrated Bishop of Tréguier; but Louis Duchesne argued that it was King Nomenoe who, in the middle of the ninth century, raised the monastery of Tréguier to the dignity of an episcopal see.
In the Tregor region there are units called Tuffs de Tréguier and Ignimbrites de Lézardrieux.
Originally established in the 5th century, the diocese was restored by the Concordat of 1801, as the combination of the dioceses of Quimper, Saint-Pol-de-Léon and Tréguier in Brittany, France.