The Panhellenion (Greek: Πανελλήνιον) or Panhellenium was a league of Greek city-states established in the year 131-132 AD by the Roman Emperor Hadrian while he was touring the Roman provinces of Greece.
Hadrian was philhellenic, and idealized the Classical past of Greece. The Panhellenion was part of this philhellenism, and was set up, with Athens at the centre, to try to recreate the apparent "unified Greece" of the 5th century BC, when the Greeks took on the Persian enemy.
The Panhellenion was primarily a religious organization, and most of the deeds of the institution which we have relate to its own self-governing. Admission to the Panhellenion was subject to scrutiny of a city's Hellenic descent.
Fighting between the delegates, however, turned the Panhellenion into an institution like the Delian League of the 5th century BC (which to some extent it was emulating) and the Panhellenion did not survive in any real sense after Hadrian's death.
From inscriptions found, member cities included Athens, Megara, Sparta, Chalcis, Argos, Acraephiae, Epidaurus, Amphicleia, Methana, Corinth, Hypata, Demetrias, Thessalonica, Magnesia on the Maeander, Eumeneia, as well the cities of Crete.
I. Romeo, “The Panhellenion and Ethnic Identity in Hadrianic Greece” ClPhil, vol. 97, 2002