Casystes or Kasystes (Ancient Greek: Κασύστης) was a port town of ancient Ionia, near Erythrae. Strabo, whose description proceeds from south to north, after describing Teos, says, "before you come to Erythrae, first is Erae, a small city of the Teians, then Corycus, a lofty mountain, and a harbour under it, Casystes; and another harbour called Erythras." It is probably the Cyssus of Livy, the port to which the fleet of Antiochus III sailed (191 BCE) before the naval engagement in which the king was defeated by Eumenes II and the Romans.
- Strabo. Geographica. Vol. p. 644. Page numbers refer to those of Isaac Casaubon's edition.
- Livy. Ab Urbe Condita Libri (History of Rome). Vol. 36.43.
- Richard Talbert, ed. (2000). Barrington Atlas of the Greek and Roman World. Princeton University Press. p. 56, and directory notes accompanying.
- Lund University. Digital Atlas of the Roman Empire.