NYMPHAIOS (or Nymphaeus) was a River-God of Bithynia and Paphlagonia in Anatolia.
The river Nymphaios flowed into the Black Sea somewhere between the Greek colony of Herakleia and the Bithynian border with Paphlagonia. It was probably the same as the River Billaeus (the historical name). The two major neighbouring rivers were the Parthenios to the east, and Saggarios in the west.
Quintus Smyrnaeus, Fall of Troy 6. 470 ff (trans. Way) (Greek epic C4th A.D.) :
"Nymphaios' stream, hard by a cave, a wide and wondrous cave : sacred it is men say, unto the Nymphai, even all that haunt the long-ridged Paphlagonian hills, and all that by full-clustered Herakleia dwell."
- Quintus Smyrnaeus, Fall of Troy - Greek Epic C4th A.D.