Poseidon (Greek: Ποσειδῶν): ancient Greek god of the sea, horses, and earthquakes. Adopting Greek mythology, the Romans identified him with Neptune.
- Perhaps an Indo-European deity (*pótis + dâwon, "ruler" + "waters"; "lord of the sea"??)
- Venerated in Mycenaean times (Po-se-da-o)
- Sanctuaries: Athens, Corinth, Isthmia, Onchestus, Poseidonia (= Paestum), Sounion
- His attribute: the trident
- Various names: Gaieochos ("earth-shaker"), Helikonios ("of the Helicon"), Hippios ("the horseman")
- An Athenian myth focused on an ancient rivalry between Poseidon and Athena: who was to give his or her name to the new city? Poseidon tried to bribe the citizens by offering them a horse; Athena gave them the olive tree, and won the contest.
- When the Romans adopted Greek mythology, they identified their Neptune with the Greek Poseidon
- In Rome, there was a basilica of Neptune in the Baths of Agrippa
- Protector of X Fretensis, XI Claudia, and XXX Ulpia Victrix