Poseidon (/pəˈsaɪdən, pɒ-, poʊ-/; Ancient Greek: Ποσειδῶν) is 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 (the terms for horses and springs are related in the Greek language). 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 Plato's Timaeus and Critias, the legendary island of Atlantis was Poseidon's domain. 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 numerous of his companions, and delaying his return by ten years.

Poseidon is famous for his contests with other deities for winning the patronage of the city. 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. In similar competitions with other deities in different cities, he causes devastating floods when he loses. Poseidon is a horrifying and avenging god and must be honoured even when he is not the patron deity of the city.

Some scholars suggested that Poseidon was probably a Pelasgian god or a god of the Minyans. However it is possible that Poseidon, like Zeus, was a common god of all Greeks from the beginning.

The earliest attested occurrence of the name, written in Linear B, is 𐀡𐀮𐀅𐀃 Po-se-da-o or 𐀡𐀮𐀅𐀺𐀚 Po-se-da-wo-ne, which correspond to Ποσειδάων (Poseidaōn) and Ποσειδάϝoνος (Poseidawοnos) in Mycenean Greek; in Homeric Greek, it appears as Ποσιδάων (Posidaōn); in Aeolic, as Ποτε(ι)δάων (Pote(i)daōn); in Doric, as Ποτειδάν (Poteidan) and Ποτειδᾶς (Poteidas); in Arcadic, as Ποσoιδᾱν (Posoidan). In inscriptions with Laconic style from Tainaron, Helos and Thuria as Ποὁιδάν (Pohoidan), indicating that the Dorians took the name from the older population. The form Ποτειδάϝων (Poteidawōn) appears in Corinth.

The origins of the name "Poseidon" are unclear and the possible etymologies are contradictive among the scholars. One theory breaks it down into an element meaning "husband" or "lord" (Greek πόσις (posis), from PIE *pótis) and another element meaning "earth" (δᾶ (da), Doric for γῆ (gē)), producing something like lord or spouse of Da, i.e. of the earth; this would link him with Demeter, "Earth-mother". Burkert finds that "the second element δᾶ- remains hopelessly ambiguous" and finds a "husband of Earth" reading "quite impossible to prove". According to Beekes in Etymological Dictionary of Greek, "there is no indication that δᾶ means 'earth'", although the root da appears in the Linear B inscription E-ne-si-da-o-ne, "earth-shaker".

Another theory interprets the second element as related to the (presumed) Doric word *δᾶϝον dâwon, "water", Proto-Indo-European *dah₂- "water" or *dʰenh₂- "to run, flow", Sanskrit दन् dā́-nu- "fluid, drop, dew" and names of rivers such as Danube (< *Danuvius) or Don. This would make *Posei-dawōn into the master of waters.

Plato in his dialogue Cratylus gives two traditional etymologies: either the sea restrained Poseidon when walking as a "foot-bond" (ποσίδεσμον), or he "knew many things" (πολλά εἰδότος or πολλά εἰδῶν).