Maillezais Cathedral (French: Cathédrale Saint-Pierre de Maillezais, or St. Peter Maillezais) is a ruined Roman Catholic church in the commune of Maillezais in the Vendée, France. Formerly the site of the Abbey of Saint-Pierre, the site grew from the 10th century abbey to the cathedral completed in the 15th century, with the many structures at the site abandoned by the end of the 17th century. Today's ruins consist of a cathedral, refectory, dormitory, kitchen, cellars, turrets and ramparts. The cathedral has been declared a heritage monument in reflection of its Romanesque, Gothic and Renaissance architectural form. It was designated a monument historique on 30 January 1924. The cathedral belonged to the Diocese of Luçon, with Roman Rites, and with St. Peter as the patron saint.