Baldassarre Cossa was Pisan antipope John XXIII (1410–1415) during the Western Schism. The Catholic Church regards him as an antipope, as he opposed Pope Gregory XII whom the Catholic Church recognizes as the rightful successor of Saint Peter. He was also an opponent of Antipope Benedict XIII, who was recognized by the French clergy and monarchy as the legitimate Pontiff.

The Council of Pisa was a controversial council held in 1409. It attempted to end the Western Schism by deposing Benedict XIII (Avignon) and Gregory XII (Rome) for schism and manifest heresy. The College of Cardinals, composed of members of both the Avignon Obedience and the Roman Obedience, who were recognized by each other and by the Council, then elected a third papal claimant, Alexander V, who lived only a few months. He was succeeded by John XXIII.