Aegimius (Greek: Αἰγίμιος) was the Greek mythological ancestor of the Dorians, who is described as their king and lawgiver at the time when they still inhabited the northern parts of Thessaly.
The Dorians inhabited the Tetrapolis, made up of the cities of Erineus, Boio, Scythian and Pindo, but Aegimius was ousted from his throne. He asked Heracles for help in a war against Coronus, son of Caeneus, king of the Lapitae who inhabited around Mount Olympus and offered him a third of his kingdom.
The Lappites were conquered, but Heracles did not take for himself the territory promised to him by Aegymius, and left it entrusted to the king who was to preserve it for Heracles' sons, the Heraclids. In gratitude, when Heracles died, Aegymius adopted Hilo, son of Heracles, as his son.
Aegymius had two sons, Dimas and Panphylus, who migrated to the Peloponnese and were considered the ancestors of the two branches of the Dorian race, the Dithians and the Panphylians of Anatolia, while the third branch, the Hylians, derive their name from Hilo, the son of Heracles, who had been adopted by Aegymius. Upon Aegymius' death, Hilo and his descendants became the kings of the Dithians.
There existed in antiquity an epic poem Aegimius of which some fragments still survive, and which is sometimes credited to Hesiod and sometimes to Cercope of Miletus. The poem, printed among hesiodic fragments, survives in less than a dozen citations, and seems to have been partly concerned with the myth of Io and Argos Panotes.