In Greek mythology, Keroessa (in Greek Κερόεσσα "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 Semystra. Keroessa was reared by Semystra 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 Χρυσόκερας) after his mother.
- History of Istanbul and Keroessa by Prof. Dr. Erendiz Ozbayoglu (in English)
- Turkish Numismatic Society Bulletin No: 33 (in Turkish; Keroessa and History of Istanbul explained, together with the Istanbul Mint)
- Foundation of Istanbul, Virtual Istanbul
- Loves of Zeus (in English)
- History of Istanbul, Istanbul Life (in English)
- Republic of Turkey Ministry of Culture and Tourism (in English)