Theseus (/ˈθiːsiːəs/; Ancient Greek: Θησεύς [tʰɛːsěu̯s]) was the mythical king of Athens and was the son of Aethra by two fathers: Aegeus and Poseidon.

Theseus was a founder-hero, like Perseus, Cadmus, or Heracles (Hercules), all of whom battled and overcame foes that were identified with an archaic religious and social order. As Heracles was the Dorian hero, Theseus was a founding hero, considered by Athenians as their own great reformer: his name comes from the same root as θεσμός ("thesmos"), Greek for "The Gathering". The myths surrounding Theseus—his journeys, exploits, and family—have provided material for fiction throughout the ages.

Theseus was responsible for the synoikismos ("dwelling together")—the political unification of Attica under Athens, represented emblematically in his journey of labours, subduing ogres and monstrous beasts. Because he was the unifying king, Theseus built and occupied a palace on the fortress of the Acropolis that may have been similar to the palace that was excavated in Mycenae. Pausanias reports that after the synoikismos, Theseus established a cult of Aphrodite Pandemos ("Aphrodite of all the People") and Peitho on the southern slope of the Acropolis.

Plutarch's vita (a literalistic biography) of Theseus makes use of varying accounts of the death of the Minotaur, Theseus' escape, and the love of Ariadne for Theseus. Plutarch's sources, not all of whose texts have survived independently, included Pherecydes (mid-fifth century BC), Demon (c. 400 BC), Philochorus, and Cleidemus (both fourth century BC).