Erechtheus in Greek Mythology was the name of one king of Athens and a secondary name for another.

In Homer's Iliad the name is applied to the earth-born son of Hephaestus later mostly called Erichthonius by later writers. Accordingly this Erichthonius is sometimes called Erechtheus I and the later king Erechtheus distinguished as Erechtheus II.

This second Erechtheus was son and heir to King Pandion I of Athens by Zeuxippe, this Pandion being son of Erichthonius / Erechtheus I.

According to Apollodorus, Erechtheus had a twin brother named Butes who married Erechtheus' daughter Chthonia. Erechtheus and Butes 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.

Erechtheus was father by his wife Praxithea of several daughers: Protogeneia, Pandora, Procris, Creusa, Oreithyia, Chthonia, and Merope some of whom have their own stories.

His reign was marked by the war between Athens and Eleusis when the Eleusians were commanded by Eumolpus of Thrace, who (acepting the most common genealogy) was son of Poseidon by Chione daugher of Boreas by Oreithyia daughter of Erechtheus and was therefore Erechtheus' own great-grandson. An oracle declared that Athens' surivial depended on the death one of the three daughters of Erechtheus. Perhaps three unmarried daughters is meant. But in one verision it is Chthonia who is sacrificed. In another both Protogeneia and Pandora, the two eldest, offer themselves up. In any case the remaining sisters, or at least some of them, are said to kill themselves. These unforunate daughters of Erechtheus became the Hyacinthides upon their death.

In the following battle between the forces of Athens and Eleusis, Erechtheus slew Eumolpus but then himself fell in battle, struck down by Poseidon's trident according to fragments of Euripides' tragedy Eumolpus. Or Zeus slew him with a thunderbolt at Poseidon's request.

Erechtheus is succeeded by Cecrops II, his brother according to a fragment from the poet Castor but his son according to Apollodorus (3.15.1).

Other sons of Erechtheus sometimes mentioned are Orneus, Metion, Pandorus, Thespius, and Eupalamus.

Poseidon in Athens was generally known as Poseidon Erechtheus and the vestibule of Poseidon's temple was named the Erechtheion.