The Land of Onias (Koine Greek: Ὀνίας) is the name given in Hellenistic Egyptian, Jewish, and Roman sources to an area in ancient Egypt's Nile Delta where a large number of Jews settled. The Land of Onias, which included the city of Leontopolis (Λεόντων πόλις), was located in the Heliopolite Nome (Hebrew: אֹן ʾOn "Heliopolis).

While accounts differ on the details, it is known that the Jews of Leontopolis had a functioning temple, distinct from and contemporary to the Temple in Jerusalem, presided over by kohenim (priests) and High Priests of the family of Onias III or Onias IV, for whom the "Land of Onias" is named. Aside from a somewhat uncertain allusion of the Hellenist Artapanus of Alexandria, only Josephus gives information about this temple.

The Talmudic accounts are internally contradictory. The establishment of a central sanctuary in Egypt was probably undertaken in response, in part, to the disorders that arose in Judea under Antiochus IV Epiphanes, the desecration and sealing up of the Second Temple under his reign and, the supplanting of the legitimate family of priests by the installation of Alcimus, the personal ambition of Onias IV, and the vast extent of the Jewish diaspora in Egypt that created a demand for a sanctuary of this nature.

at its pinnacle Onias’ Temple was a major religious and cultural center of Egyptian Judaism. This assumption is bolstered by the circumstance that several works of Jewish-Hellenistic literature such as 3 Maccabees, Pseudo-Hecataeus, Joseph & Aseneth, and several oracles of the Third Book of the Sibylline Oracles appear to have been written in the milieu of the Oniad Temple.

— Meron-Martin Piotrkowski, Priests in Exile

The account of Josephus in The Jewish War refers to the Onias who built the Temple at Leontopolis as "the son of Simon", implying that it was Onias III, and not his son, who fled to Egypt and built the Temple. This account, however, is contradicted by the story that Onias III was murdered in Antioch in 171 BCE. Josephus' account in the Antiquities is considered by some to be more probable, namely, that the builder of the temple was a son of the murdered Onias III and that, a mere youth at the time of his father's death, he had fled to the court of Alexandria in consequence of the Syrian persecutions, perhaps because he thought that salvation would come to his people from Egypt. Others contend that the story of Onias III's murder is of dubious historical accuracy, instead being based on the story of the murder of Antiochus IV Epiphanes’ nephew, which actually occurred, in part because it is difficult to believe Antiochus IV would be heartbroken at the death of Onias III (a political opponent), as the story of his murder in 2 Maccabees claims, and in part because the stories are very similar. 2 Maccabees seems to have instead possibly, assigned the story of Onias III's flight to Egypt to a character named "Philip" for thematic reasons, as evidenced by there inexplicably being two "Philips" that were close to Antiochus in the narrative that cannot be the same person, an act which implies the author knew of the story of Onias III's flight to Egypt, and similarly made Onias into a martyr for primarily thematic purposes. Notably, Theodore of Mopsuestia's Commentary on the Psalms 54(55) rejects the Maccabean narrative and states that Onias III fled to Egypt. He uses this account as a source for the rest of the passage, possibly having learned of it from a piece of Rabbinic literature. Theodore seems to consider 2 Maccabees to be less reliable because he lived in the area where the alleged murder took place, and there is a lack of evidence suggesting that the city was aware of the event.

Ptolemy VI Philometor was pharaoh at that time. He probably had not yet given up his claims to Coele-Syria and Judea and gladly gave refuge to such a prominent personage of the neighboring country. Onias now requested the king and his sister-wife Cleopatra II allow him to build a sanctuary in Egypt similar to the one at Jerusalem, where he would employ Levites and priests of his clan; he referred to the prediction of Isaiah 19:19 that a temple would be erected in Egypt.

According to Josephus, the temple of Leontopolis existed for 343 years but the general opinion is that this number must be changed to 243. He relates that the Roman emperor Vespasian feared that through this temple, Egypt might become a new center for Jewish–Roman wars and therefore ordered the governor of Egypt, Tiberius Julius Lupus, to demolish it. Lupus died in the process of carrying out the order, and the task of stripping the temple of its treasures, barring access to it, and removing all traces of divine worship at the site was completed by his successor, Gaius Valerius Paulinus, which dates the event to c. March–August 73.