The Rioni or Rion River (Georgian: რიონი Rioni,Greek: Φᾶσις Phasis) is the main river of western Georgia. It originates in the Caucasus Mountains, in the region of Racha and flows west to the Black Sea, entering it north of the city of Poti (near ancient Phasis). The city of Kutaisi, once ancient city of Colchis, lies on its banks.

Known to the ancient Greeks as the Phasis River, Rioni was first mentioned by Hesiod in his Theogony (l.340); later writers like Apollonius Rhodius (Argonautica 2.12.61), Virgil (Georgics 4.367) and Aelius Aristides (Ad Romam 82) considered it the easternmost limit of the navigable seas. Socrates, in Phaedo referred to the portion of the world he knew of as between the Pillars of Hercules and the River Phasis, while Herodotus considered Rioni as a boundary between Europe and Asia

The term "pheasant" and the scientific name Phasianus colchicus are derived from "Phasis" and "Colchis", as this was said to be the region from which the Common Pheasant was introduced to Europe in ancient times (the ring-necked pheasants seen in the present day were later introduced from East Asia; see Common Pheasant for details).