Tréguier Cathedral (Cathédrale Saint-Tugdual de Tréguier) is a Roman Catholic church and former cathedral in Tréguier in Brittany, dedicated to Saint Tudwal. It was formerly the seat of the Bishopric of Tréguier, abolished under the Concordat of 1801, when its territories were divided between the Diocese of Quimper and the Diocese of Saint-Brieuc, known since 1852 as Saint-Brieuc-Tréguier. John VI, Duke of Brittany is buried here.