The Peiros (Greek: Πείρος, formerly also Kamenitza and Achelous[1]) is a river in the central and the northwestern parts of Achaea, Greece.

[2] It rises in the northeastern part of the Erymanthos mountains, near the village Lakkomata.

It passes through the municipal units of Farres, Olenia and Dymi.

It empties into the Gulf of Patras between the towns Alissos and Kato Achaia.

Two ancient cities were situated near the river: Olenos and Pharae.