The Serapeum of Alexandria in Ptolemaic Egypt was a temple built by Ptolemy III (reigned 246222 BCE) and dedicated to Serapis, the syncretic Hellenistic-Egyptian god who was made the protector of Alexandria. By all detailed accounts, the Serapeum was the largest and most magnificent of all temples in the Greek quarter of Alexandria. Besides the image of the god, the temple precinct housed an offshoot collection of the great Library of Alexandria. The geographer Strabo tells that this stood in the west of the city. Nothing now remains above ground.
Excavations at the site of the column of Diocletian in 1944 yielded the foundation deposits of the Temple of Serapis. These are two sets of ten plaques, one each of gold, of silver, of bronze, of faience, of sun-dried Nile mud, and five of opaque glass. The inscription that Ptolemy III Euergetes built the Serapeion, in Greek and hieroglyphs, marks all plaques; evidence suggests that Parmeniskos was assigned as architect. The foundation deposits of a temple dedicated to Harpocrates from the reign of Ptolemy IV were also found within the enclosure walls.
Subterranean galleries beneath the temple were most probably the site of the mysteries of Serapis. In 1895, a black diorite statue representing Serapis in his Apis bull incarnation with the sun-disk between his horns was found at the site; an inscription dates it to the reign of Hadrian (117-38).