Serapis is a Graeco-Egyptian god. The cult of Serapis was introduced during the 3rd century BC on the orders of Ptolemy I of Egypt as a means to unify the Greeks and Egyptians in his realm. The god was depicted as Greek in appearance, but with Egyptian trappings, and combined iconography from a great many cults, signifying both abundance and resurrection. A serapeum was any temple or religious precinct devoted to Serapis. The cultus of Serapis was spread as a matter of deliberate policy by the Ptolemaic kings, who also built an immense serapeum in Alexandria.

However, there is evidence which implies that cult of Serapis existed before the Ptolemies came to power in Alexandria – a temple of Sarapis (or Roman Serapis) in Egypt is mentioned in 323 BC by both Plutarch (Life of Alexander, 76) and Arrian (Anabasis, VII, 26, 2). The common assertion that Ptolemy "created" the deity is derived from sources which describe him erecting a statue of Sarapis in Alexandria: this statue enriched the texture of the Sarapis conception by portraying him in both Egyptian and Greek style. Though Ptolemy I may have created the cult of Sarapis and endorsed him as a patron of the Ptolemaic dynasty and Alexandria, Sarapis was a syncretistic deity derived from the worship of the Egyptian Osiris and Apis (Osiris + Apis = Oserapis/Sarapis) and also gained attributes from other deities, such as chthonic powers linked to the Greek Hades and Demeter, and benevolence linked to Dionysus.

Serapis continued to increase in popularity during the Roman period, often replacing Osiris as the consort of Isis in temples outside Egypt. In 389, a Christian mob led by the Patriarch Theophilus of Alexandria destroyed the Alexandrian serapeum, but the cult survived until all forms of pagan religion were suppressed under Theodosius I in 391.