Erechtheus (/ᵻˈrɛkθjuːs, -θiəs/; Ancient Greek: Ἐρεχθεύς) in Greek mythology was the name of an archaic king of Athens, the founder of the polis and, in his role as god, attached to Poseidon, as "Poseidon Erechtheus". The mythic Erechtheus and an historical Erechtheus were fused into one character in Euripides' lost tragedy Erechtheus (423/22 BCE). The name Erichthonius is carried by a son of Erechtheus, but Plutarch conflated the two names in the myth of the begetting of Erechtheus.
Athenians thought of themselves as Erechtheidai, the "sons of Erechtheus". In Homer's Iliad (2. 547–48) he is the son of "grain-giving Earth", reared by Athena. The earth-born son was sired by Hephaestus, whose semen Athena wiped from her thigh with a fillet of wool cast to earth, by which Gaia was made pregnant.
In the contest for patronship of Athens between Poseidon and Athena, the salt spring on the Acropolis where Poseidon's trident struck was known as the sea of Erechtheus.
The central gods of the Acropolis of Athens were Poseidon Erechtheus and Athena Polias, "Athena patron-guardian of the city". The Odyssey (VII.81) already records that Athena returned to Athens and "entered the strong-built house of Erechtheus". The archaic joint temple built upon the spot that was identified as the Kekropion, the hero-grave of the mythic founder-king Cecrops and the serpent that embodied his spirit was destroyed by the Persian forces in 480 BC, during the Greco-Persian wars, and was replaced between 421 and 407 BCE by the present Erechtheum. Continuity of the site made sacred by the presence of Cecrops is inherent in the reference in Nonnus' Dionysiaca to the Erechtheion lamp as "the lamp of Cecrops". Priests of the Erechtheum and the priestess of Athena jointly took part in the procession to Skiron that inaugurated the Skira festival near the end of the Athenian year. Their object was the temenos at Skiron of the hero-seer Skiros, who had aided Eumolpus in the war between Athens and Eleusis in which Erechtheus II, the hero-king, was both triumphant and died.