Greco-Egyptian deity of the sun first encountered at Memphis, where his cult was celebrated in association with that of the sacred Egyptian bull Apis (who was called Osorapis when deceased). He was thus originally a god of the underworld but was reintroduced as a new deity with many Hellenic aspects by Ptolemy I Soter (reigned 305–284 BC), who centred the worship of the deity at Alexandria.

The Serapeum at Alexandria was the largest and best known of the god's temples. The cult statue there represented Serapis as a robed and bearded figure regally enthroned, his right hand resting on Cerberus (the three-headed dog who guards the gate of the underworld), while his left held an upraised sceptre. Gradually Serapis became revered not only as a sun god (“ Zeus Serapis”) but also as a lord of healing and of fertility. His worship was established in Rome and throughout the Mediterranean, following the trade routes and being particularly prominent in the great commercial cities. Among the Gnostics (early Christian heretics who believed that matter is evil and the spirit is good; see Gnosticism) he was a symbol of the universal godhead. The destruction of the Serapeum at Alexandria by the patriarch Theophilus (Theophilus of Alexandria, Saint) and his followers in AD 391 signaled the final triumph of Christianity not only in Egypt but throughout the Roman Empire.