The chief center of the worship of Serapis in Ptolemaic times was Alexandria, where it was established, according to tradition, by Ptolemy Soter. This great ruler of Egypt appears to have wished to find some god who could be worshipped both by Greeks and Egyptians at a common shrine, and one whom he could cause to be regarded as the characteristic god of his dynasty in Egypt. The most important Egyptian god at the time was Osiris, that is to say Osiris-Apis, the great god of the Egyptian Underworld, but it was impossible for him to remove the great sanctuary of this god, and he therefore determined either to rebuild some ruined Serapeum at Alexandria, or to found a new one wherein he might set up a statue which should be worshipped both as the god of the Egytian Underworld and the Greek Hades, and in which would be united the attributes of Osiris Khent Amenti, and of Dis. Whilst Ptolemy was meditating upon these or similar things he had a dream, wherein a colosssal statue of some god appeared to him, and told him to remove it from where it was to Alexandria; according to Plutarch (De Iside et Osiride, 28), he had never seen a similar statue, and he knew neither the place where it stood, nor to whom it belonged. One day he happened to mention his dream to Sosibius, and described the statue which he had seen, whereon this man declared that he had seen a statue like it at Sinope. Tradition says that this was Sinope on the Pontus, and adds that as the inhabitants of the city were extremely unwilling to part with their statue, it, of its own accord, after waiting for three years, entered into a ship and arrived at Alexandria safely after a voyage of only three days. When the Greeks came to see the statue it was introduced to them as the god Hades, and the Egyptian priets were ready to bestow upon him the name Asar-Hapi, or Serapis, by which name the Greeks were, apparently, quite contented to call him. Thus both the Greeks and Egyptians in Alexandria acquired a god whom they willingly worshipped as the god of the Underworld.

As soon as the god who was now called Serapis had been established in his new home, his fomer worship and rites were greatly modified, and his services and processions were made to resemble those of the Egyptians, who naturally expected their main features to be brought into harmony with those of the cult of Osiris, their national god. It appears to have been to the interest of all parties to welcome Serapis, and all must admire the astute action of Ptolemy, who succeeded in making the Greeks think that in worshipping this god they were adoring one of their own native deities, and who persuaded the Egyptians that they were maintaining the supremacy of Osiris-Apis in spite of the fact that the Macedonians were the rulers and masters of the country. Some doubt has been cast upon the identification of the Sinope mentioned by Plutarch with the Sinope of Pontus, but with insufficient reason. The Serapeum which Ptolemy repaired, or founded, was probably near Raqetit, and was a very remarkable building; its main plan seems to have resembled that of the famous Serapeum at Memphis, but parts of it were richly painted and gilded, and it possessed a fine library which was said to contain some 300,000 volumes. The following is Plutarch's account of the introduction of the god of Sinope into Egypt: