Psyllium or Psyllion (Ancient Greek: Ψύλλιον), or Psylleium or Psylleion (Ψύλλειον), or Psillium or Psillion, or Psylla (Ψύλλα), was a fortified emporium on the coast of ancient Bithynia located on the Pontus Euxinus between Crenides and Tium. The Tabula Peutingeriana erroneously calls it Scylleum.

Its site is located near Ağva in Asiatic Turkey.