Gaius Julius Callistus (flourished 1st century) was a Greek imperial freedman during the reigns of Roman Emperors Caligula and Claudius.
He is named by the ancient sources as a prime mover in Caligula's murder (24 January 41) rallying the other freedmen to the cause.
After the disgrace and death of Roman Empress Valeria Messalina, Callistus is said to have promoted Caligula's former wife Lollia Paulina as a replacement.
Most of the details of Callistus' personal life and ultimate fate are unknown, although he was dead by the end of Claudius' reign.
Callistus' grandchild Nymphidius Sabinus later claimed to be the product of this union, and tried unsuccessfully to gain the principate in the Year of the Four Emperors.