Guillaume Fillastre (the Elder) (b. 1348 at La Suze, Maine, France; d. Rome, 6 November 1428) was a French Cardinal, canonist, humanist, and geographer.
After graduating as doctor juris utriusque, Fillastre taught jurisprudence at Reims, and in 1392 was appointed dean of its metropolitan chapter. During the Western Schism he showed at first much sympathy for Benedict XIII (Peter de Luna). In 1409, however, he took part in the attempt to reconcile the factions at the Council of Pisa. Antipope John XXIII conferred on him and his friend Pierre d'Ailly the dignity of cardinal (1411), and in 1413 he was made Archbishop of Aix. Fillastre took a very important part in the Council of Constance, where he and Cardinal d'Ailly were the first to agitate the question of the abdication of the rival claimants (February, 1415). He gained distinction through the many legal questions on which he gave decisions. Pope Martin V, in whose election he had been an important factor, appointed him legatus a latere to France (1418), where he was to promote the cause of Church unity. In recognition of his successful efforts in this capacity, he was made Archpriest of the Lateran Basilica. In 1421 he resigned the See of Aix, and in 1422 was assigned to the See of Saint-Pons-de-Thomières. He died at Rome in his eightieth year, as Cardinal-Priest of San Marco.