In Greek mythology, Keroessa (in Greek "the horned") was a heroine of the foundational myth of Byzantium. She was the daughter of Io and Zeus; elder sister of Epaphus; and mother of Byzas, founder of Byzantium, with her uncle, Poseidon.
According to the historian Hesychius of Miletus, as Io, changed into a heifer and being chased by a gadfly on behalf of the jealous Hera, was passing through Thrace, she gave birth to a girl, Keroessa, on the banks of the Golden Horn, by the altar of the nymph Semestra. Keroessa was reared by Semestra and grew up surpassing other local maidens in beauty. She had intercourse with Poseidon and in due course gave birth to a son, whom she named Byzas. He became the founder of Byzantium and named the Golden Horn (Greek ?) after his mother.