Callixtus III (also Calixtus III or Callistus III; died between 1180 and 1184<!--19 October 1183-->) was an antipope from September 1168 until his resignation in August 1178. He was the third antipope elected in opposition to Pope Alexander III during the latter's struggle with the Emperor Frederick Barbarossa.
Callixtus' baptismal name was John. He entered the Vallombrosan monastery of Struma near Arezzo as a boy. In November 1158, the emperor placed the Vallombrosan under imperial protection. By that time, John was the abbot of Struma and one of the most important supporters of the emperor in Tuscany. After the disputed papal election of 1159, he supported the imperial candidate Victor IV over Alexander III. He was rewarded by Victor with the cardinal-bishopric of Albano, but was not immediately consecrated, rather continuing on as abbot.
After the death of Victor's successor, Paschal III, on 20 September 1168, his supporters gathered in Rome to elect a new (anti-)pope. John was chosen that same month. He took the name Callixtus and sent Umfredo, (pseudo-)cardinal of Santa Susanna, to Germany to inform Frederick of his election. According to Cardinal Boso of Santa Pudenziana, a partisan of Alexander III, Frederick was "inwardly deeply grieved" by the election of Callixtus, whom he had never met. Nevertheless, Frederick expressed his intention to go with an army to Italy in 1170 to restore the unity of the Church. No such expedition took place, however.
Alexander appointed Callixtus rector of Benevento. He still held that position in 1180, but by 1184 he was dead.