Antipope Callixtus III or Callistus III (died before October 19, 1183) was Antipope from September 1168 to 29 August 1178. His real name was Giovanni, Abbot of Struma. He was a strong and early supporter of Frederick Barbarossa, and also supported Antipope Victor IV, who made him a cardinal-bishop of Albano. He was named successor to Antipope Paschal III by a small number of schismatic cardinals. Callixtus was primarily a bargaining chip of Frederick to pressure Alexander III, and he had only limited geographical support. He resided in Viterbo.