In Greek mythology, Keroessa (in Greek Κερόεσσα) was a nymph (νύμφη) or Princess of Byzantium, on the Bosphorus Strait separating Europe and Asia. She was the daughter of Io and Zeus; and mother of Byzas the Megarian, founder of Byzantium, with her uncle, Poseidon.
Zeus fell in love with Io, the daughter of Inachus, King of the City of Argos and God of the River of Argos. The King of the Gods temporarily transformed his beloved Io into a heifer in order to protect her from the wrath of his wife, Hera, Queen of the Gods. In her wanderings, Io crossed the Bosphorus, giving the strait its name (boos-foros, which is Greek for cow-ford). After reassuming her original form, Io gave birth to a girl, Keroessa, on the banks of the Golden Horn. Keroessa was brought up by the nymph Semestra and in due course she gave birth to a son, whom she named Byzas, of the sea god Poseidon, grandson of Gaea (Mother Earth) and Uranus (Father Sky), son of Cronus, elder brother of Zeus and sovereign deity of all waters from the Pillars of Hercules to the Hellespont. Keroessa's son, Byzas the Megarian, in time became the founder of Byzantium and named the Golden Horn (Greek Χρυσοκερας Khrysokeras or Chrysoceras ) after his mother.