Aegium or Aigion (Ancient Greek: Αἴγιον), or Aegeium or Aigeion (Αἴγειον), was a town and polis (city-state) of ancient Achaea, and one of the twelve Achaean cities. It was situated upon the coast west of the river Selinus, 30 stadia from Rhypae, and 40 stadia from Helice.
The city stood between two promontories in the corner of a bay, which formed the best harbour in Achaea next to that of Patrae. It is said to have been formed out of a union of seven or eight villages. It was already mentioned in the Homeric Catalogue of Ships in the Iliad. When the neighbouring city of Helice sank into the sea following an earthquake in 373Â BCE, Aegium annexed its territory and became the chief city of the Achaean League. When the League dissolved later in the same century, however, Aegium came for some time under Macedonian rule.