Aegeus was one of the kings of Athens in Greek mythology, who gave his name to the Aegean Sea, was the father of Theseus, and founded Athenian institutions.

The patronymic Aegeides was formed from his name, and was especially used to designate his son Theseus.

Aegeus was the son of Pandion II, king of Athens, and Pylia, daughter of King Pylas of Megara. He was brother to Pallas, Nysus, Lykos, and the wife of Sciron. Some accounts instead suggest that he was the son of Scyrius or Phemius and only an adopted son of Pandion, and so not truly of the Erechtheid lineage.

Aegeus' first wife was Meta, daughter of Hoples, and his second wife was Chalciope, daughter of Rhexenor; neither bore him children. He was credited as the father of Medus by the witch Medea. In rare accounts, Pallas was also said to be his son, and Aegeus was likewise said to have fathered Megareus, the eponymous founder of Megara.

Reign — Aegeus was born in Megara, where his father Pandion II had settled after being expelled from Athens by the sons of Metion. After Pandion's death, Aegeus and his three brothers attacked Athens, took control, and expelled the usurpers. They divided power among themselves: Megara was then part of Attica, so Nisus became king of that city, Lycus became king of Euboea, and Pallas received the southern territory, while Aegeus, as eldest brother, obtained sovereignty of Attica and the best part, Athens. Later Lycus was driven from the territory by Aegeus and sought refuge in Arene in Messenia under King Aphareus, and Pallas and his fifty sons revolted but were crushed by Theseus.

Heirless King — Without a male heir from his previous marriages, Aegeus consulted the oracle at Delphi. According to Pausanias, he attributed his misfortune to Aphrodite's anger and introduced her worship as Aphrodite Urania at Athens. The oracle's cryptic words were: 'Do not loosen the bulging mouth of the wineskin until you have reached the height of Athens, lest you die of grief.' Not understanding, Aegeus visited Pittheus, king of Troezen, renowned for expounding oracles. Pittheus grasped the prophecy and, when Aegeus was drunk, introduced him to his daughter Aethra; they lay together, and in some versions Aethra afterwards bedded Poseidon on the island of Sphairia. When Aethra became pregnant, Aegeus buried his sandal, shield, and sword beneath a great rock, instructing that when their son grew up he should lift the rock and bring the tokens to claim his birthright, and then returned to Athens.

On his return Aegeus married Medea, who had fled Corinth and the wrath of Jason, and they had a son, Medus. When Theseus grew up he found the tokens and came to Athens to claim his inheritance; Aegeus recognized him by the sword, shield, and sandals. Medea, seeing Theseus as a threat to her own child's prospects, first tried to discredit him and then to poison him, and when Aegeus discovered the schemes he drove Medea from Athens.

Conflict with Crete — While visiting Athens, Androgeus, son of King Minos, defeated Aegeus's men in every contest at the Panathenaic Games. Out of envy Aegeus sent him against the Marathonian Bull, which killed him. Minos declared war on Athens but offered peace on the condition that Athens send seven youths and seven maidens every nine years to Crete to be devoured by the Minotaur. This continued until Theseus killed the Minotaur with the help of Ariadne.

Theseus and the Minotaur — Theseus, who grew up in Troezen, lifted the rock and took his father's weapons; his mother told him his father's identity and bade him return the tokens to Athens. He chose the dangerous land route and arrived without revealing himself, and Aegeus received him with suspicion. Medea tried to have Theseus killed, first by urging Aegeus to send him against the Marathonian Bull, which he overcame, then by poison; but Aegeus recognized his son at the last moment and dashed the poisoned cup from his hand, and Medea was sent away. Before Theseus departed for Crete, Aegeus told him to hoist white sails on his return if he succeeded in slaying the Minotaur. Theseus forgot the instruction, and when Aegeus saw the ships approaching without white sails he assumed his son was dead and threw himself to his death — according to some sources from the Acropolis or an unnamed rock, and according to Latin authors into the sea, thereafter called the Aegean Sea.

Sophocles' tragedy Aegeus has been lost, but Aegeus features in Euripides' play Medea.

At Athens the traveler Pausanias was informed in the second century CE that the cult of Aphrodite Urania above the Kerameikos was so ancient that it had been established by Aegeus, whose sisters were barren and who was himself childless at the time.

There was also a heroon, or shrine, of Aegeus in Athens, called the Aigeion.