Poseidon (; Greek: Ποσειδῶν) was one of the Twelve Olympians in ancient Greek religion and mythology, presiding over the sea, storms, earthquakes and horses. He was the protector of seafarers and the guardian of many Hellenic cities and colonies. In pre-Olympian Bronze Age Greece, Poseidon was venerated as a chief deity at Pylos and Thebes, with the cult title "earth shaker"; in the myths of isolated Arcadia, he is related to Demeter and Persephone and was venerated as a horse, and as a god of the waters. Poseidon maintained both associations among most Greeks: He was regarded as the tamer or father of horses, who, with a strike of his trident, created springs (in the Greek language, the terms for both are related). His Roman equivalent is Neptune.

Homer and Hesiod suggest that Poseidon became lord of the sea when, following the overthrow of his father Cronus, the world was divided by lot among Cronus' three sons; Zeus was given the sky, Hades the underworld, and Poseidon the sea, with the Earth and Mount Olympus belonging to all three. In Homer's Iliad, Poseidon supports the Greeks against the Trojans during the Trojan War; in the Odyssey, during the sea-voyage from Troy back home to Ithaca, the Greek hero Odysseus provokes Poseidon's fury by blinding his son, the Cyclops Polyphemus, resulting in Poseidon punishing him with storms, causing the complete loss of his ship and companions, and delaying his return by ten years. Poseidon is also the subject of a Homeric hymn. In Plato's Timaeus and Critias, the legendary island of Atlantis was Poseidon's domain.

According to legend, Athena became the patron goddess of the city of Athens after a competition with Poseidon, though he remained on the Acropolis in the form of his surrogate, Erechtheus. After the fight, Poseidon sent a monstrous flood to the Attic plain to punish the Athenians for not choosing him.