Antipope John XXIII
Baldassarre Cossa (c. 1370 – 22 December 1419) 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 now recognizes as the rightful successor of Saint Peter. He was also an opponent of Antipope Benedict XIII, who was recognized by the French bishop as legitimate Pontiff.

Cossa was born in the Kingdom of Naples. In 1403, he served as a papal legate in Romagna. He participated in the Council of Pisa in 1408, which sought to end the Western Schism with the election of a third alternative pope. In 1410, he succeeded Antipope Alexander V, taking the name John XXIII. At the instigation of Sigismund, King of the Romans, Pope John called the Council of Constance of 1413, which deposed John XXIII and Benedict XIII, accepted Gregory XII's resignation, and elected Pope Martin V to replace them, thus ending the schism. John XXIII was tried for various crimes, though later accounts question the veracity of those accusations. Towards the end of his life Cossa restored his relationship with the Church and was made Cardinal Bishop of Frascati by Pope Martin V.