Aeacus was a king of Aegina and a demigod son of Zeus.
Aeacus was son of Zeus and Aegina, a daughter of the river-god Asopus. He was born on the island of Oenone, to which Aegina had been carried by Zeus to secure her from the anger of her parents, and whence this island was afterwards called Aegina.
At the time Aeacus was born, Aegina was not yet inhabited, so Zeus changed the ants of the island into men (Myrmidons) over whom Aeacus ruled, or that he made men grow up out of the earth. Aeacus while he reigned in Aegina was renowned in all Greece for his justice and piety, and was frequently called upon to settle disputes not only among men, but even among the gods themselves.
He was such a favourite with the latter, that, when Greece was visited by a drought in consequence of a murder which had been committed, the oracle of Delphi declared that the calamity would not cease unless Aeacus prayed to the gods that it might. Aeacus prayed, and it ceased in consequence. Aeacus himself showed his gratitude by erecting a temple to Zeus on mount Panhellenion, and the Aeginetans afterwards built a sanctuary in their island called Aeaceum, which was a square place enclosed by walls of white marble. Aeacus was believed in later times to be buried under the altar in this sacred enclosure.
A legend preserved in Pindar relates that Apollo and Poseidon took Aeacus as their assistant in building the walls of Troy. When the work was completed, three dragons rushed against the wall, and while the two of them which attacked those parts of the wall built by the gods fell down dead, the third forced its way into the city through the part built by Aeacus. Hereupon Apollo prophesied that Troy would fall through the hands of Aeacus's descendants, the Aeacidae.
Aeacus was also believed by the Aeginetans to have surrounded their island with high cliffs to protect it against pirates.
By Endeïs Aeacus had two sons, Telamon (father of Ajax and Teucer) and Peleus (father of Achilles), and by Psamathe a son, Phocus, whom he preferred to the two others, both of whom contrived to kill Phocus during a contest, and then fled from their native island.
After his death, Aeacus became (along with the Cretan brothers Rhadamanthus and Minos) one of the three judges in Hades.
Alexander the Great traced his ancestry (through his mother) to Aeacus.