His rival is Athena.
But probably his enemy is Oceanus, the titan of the sea.
Poseidon's greatest enemy was his father, Cronus.
The reason they were enemies is because of the war that Poseidon started.
Zeus is Poseidon's enemy and brother.
Poseidon
athena is his enemy
Yes
Odysseus blinded Poseidon's son Polyphemus.Odysseus did not consider Poseidon his enemy, as it would be foolhardy for a mortal to be an enemy to any god. However, Poseidon was a wrathful god who was punishing Odysseus for blinding his son, the cyclops Polyphemus, by piercing his only eye. As a result, Poseidon hampered Odysseus' efforts to get home, giving him trials.Eventually Odysseus builds a shrine to Poseidon to appease the god, although the actual event is takes place after the events in the Odyssey.Poseidon is not a traditional enemy, in the normal sense of 'enemy'.
Poseidon
Poseidon was Athena's enemy:(
Trojan warriors
Because Cronus is a Titan, which are enemies with the Greek Gods (which includes Poseidon, Zeus, Hades, etc...), Cronus was naturally an enemy with Poseidon due to their sort of never-ending war.
They competed over th city Athens