Erechtheus (/ɪˈrɛkθjuːs, -θiəs/; Ancient Greek: Ἐρεχθεύς) in Greek mythology was a king of Athens, the founder of the polis and, in his role as god, attached to Poseidon, as "Poseidon Erechtheus". 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) Erechtheus 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 patronage 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 second Erechtheus was given a historicizing genealogy as son and heir to King Pandion I of Athens by Zeuxippe, this Pandion being son of Erichthonius. This later king Erechtheus may be distinguished as Erechtheus II. His siblings were Philomela, Procne, Butes and possibly Teuthras.

Erechtheus was father, by his wife Praxithea, of sons: Cecrops, Pandorus, Metion and of six daughters, the eldest was Protogeneia, Pandora, Procris, Creusa, Oreithyia and Chthonia. Sometimes, his other mentioned children were Orneus, Thespius, Eupalamus, Sicyon and Merope.

According to Pseudo-Apollodorus, Erechtheus II had a twin brother named Butes who married Erechtheus' daughter Chthonia, the "earth-born". The brothers divided the royal power possessed by Pandion, Erechtheus taking the physical rule but Butes taking the priesthood of Athena and Poseidon, this right being passed on to his descendants. This late origin myth or aition justified and validated the descent of the hereditary priesthood of the Boutidai family.

His reign was marked by the war between Athens and Eleusis, when the Eleusinians were commanded by Eumolpus, coming from Thrace. An oracle declared that Athens' survival depended on the death of one of the three daughters of Erechtheus. Perhaps this means the three unmarried daughters. In one version it is Chthonia, the youngest, whom he sacrifices. In another, it is both Protogeneia and Pandora, the two eldest, who offer themselves up. In any case the remaining sisters (excepting Orithyia who had been kidnapped by Boreas), or at least some of them, are said to kill themselves. The story of the unfortunate daughters of Erechtheus is comparable to those of the daughters of Hyacinthus of Lacedaemon, and of the daughters of Leos.

In the following battle between the forces of Athens and Eleusis, Erechtheus won the battle and slew Eumolpus, but then himself fell, struck down by Poseidon's trident. According to fragments of Euripides' tragedy Erechtheus, Poseidon avenged his son Eumolpus' death by driving Erechtheus into the earth with blows of his trident,