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).

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.

It is said that "the failure of Colchis to emerge as a strong kingdom or to be maintained as a province of Rome has been blamed on the pestilential climate of the Phasis Valley, a situation remarked upon by travelers down to modern times, when the swamps were finally drained.

[6] It starts on the southern slopes of the Caucasus Mountains at 2,960 metres (9,710 ft) above sea level, north of the town of Oni.

Stephanus of Byzantium wrote that there was also another river which was named Phasis, in Taprobana (Ancient Greek: Φᾶσις ἐν τῇ Ταπροβάνῃ) (Ceylon or Sri Lanka).