The Panionium was an Ionian sanctuary dedicated to Poseidon Helikonios and the meeting place of the Ionian League. It was on the peninsula of Mt. Mycale, about 100 kilometres (62 mi) south of Smyrna—now İzmir, in Turkey. Herodotus describes it as follows:
The Panionion is a sacred ground in Mykale, facing north; it was set apart for Poseidon of Helicon by the joint will of the Ionians. Mykale is a western promontory of the mainland opposite Samos; the Ionians used to assemble there from their cities and keep the festival to which they gave the name of Panionia.
The sanctuary was under the control of the Ionian city of Priene, one of the twelve cities comprising the Ionian League. Priene was about 15 kilometres (9 mi) away, on the opposite side of Mt. Mycale. The Prienians managed the sanctuary and presided at the sacrifices and sacred rites.
The Panionium was the site of the Ionian religious festival and games (panegyris) called the Panionia. Under Persian rule, activities at the Panionium were curtailed. Writing at the end of the 5th century BC, Thucydides says that the Ionians were then celebrating their festival at Ephesus. Diodorus writes that the Ionians were forced to move the Panionia from the Panionium to Ephesus, because of war in the surrounding area. Under Alexander the Great the games and festival were again held at the Panionium, and continued to be so under Roman rule, without however, regaining their previous importance.