Callixtus III, né Alphonso de Borgia (December 31, 1378 - August 6, 1458) was born in Xàtiva, Valencia, Spain and was pope from April 8, 1455 to August 6, 1458. His early career was spent as a professor of law at Lleida and then as a diplomat in the service of the kings of Aragon, especially during the Council of Basel. He became a cardinal after reconciling Pope Eugenius IV with King Alfonso V of Aragon.

He was raised to the papal chair in 1455 at a very advanced age as a compromise candidate. He was feeble and incompetent. The great object of his policy was the urging of a crusade against the Turks, who had captured Constantinople in 1453, but he did not find the Christian princes responsive to his call despite his every effort.

He made two of his nephews cardinals, one of whom, Rodrigo Borgia, later became the notably corrupt Pope Alexander VI.

He ordered a new trial for Joan of Arc, at which she was posthumously vindicated. He died in 1458.

According to one story, first appearing in a posthumous biography in 1475 and later embellished and popularized by Pierre-Simon Laplace, Callixtus excommunicated the 1456 apparition of Halley's Comet, believing it to be an ill omen for the Christian defenders of Belgrade, who were at that time being besieged by the armies of the Ottoman empire. No known primary source supports the authenticity of this account. Callixtus' papal bull of June 29, 1456, which called for public prayer for the success of the crusade, makes no mention of the comet, and by August 6, when the Turkish siege was broken, the comet not been visible for several weeks.