Callistus, Calistus, Callixtus, and Calixtus (all four in Latin) and Kallistos (in Greek; meaning the most beautiful one) may refer to:
Popes and antipopes
Saint Callixtus I, pope from c. 217 to 222
Pope Callixtus II, pope from 1119 to 1124
Antipope Callixtus III, antipope from 1168 to 1178
Pope Callixtus III, pope from 1455 to 1458
Other persons
Callistus, a Roman general of the 3rd century more commonly known as Balista
Patriarch Callistus I of Constantinople
Patriarch Callistus II of Constantinople
Callistus Caravario (1903–1930), Italian Roman Catholic priest and missionary
Callistus Chukwu (born 1990), Nigerian footballer
Callistus Ndlovu (1936–2019), Zimbabwean politician
Callistus Valentine Onaga (born 1958), Nigerian Roman Catholic bishop
Callistus Rubaramira (born 1950), Ugandan Roman Catholic bishop
Gaius Julius Callistus, a Greek freedman of the Roman emperor Caligula
Georg Calixtus (1586–1656), German Lutheran theologian
Kallistos Ware (1934–2022), Metropolitan of Diokleia in Phrygia, an Orthodox assistant bishop in the UK Nikephoros Kallistos Xanthopoulos, last of the Greek ecclesiastical historians, flourished c. 1320