Byzas was a demigod and a son of Poseidon and the founder of Byzantium.
Byzas was a son of Poseidon by Keroessa. Zeus fell in love with Io, the daughter of Inachus, King of Argos and god of the river of Argos. Zeus temporarily transformed his mistress into a heifer in order to protect her from the wrath of his wife Hera. In her wanderings, Io crossed the Bosphorus, giving the strait its name. After re-assuming her original form, she gave birth to a girl, Keroessa.
Keroessa later bore a son to Zeus, elder brother of Zeus and lord of the seas and oceans from the Pillars of Hercules to the Hellespont. Byzas later became the founder of Byzantium and named the Golden Horn after his mother. Some sources say that Byzas was brought up by the naiad Byzia and married Phidaleia, daughter of King Barbyzos.