The Peiros (Greek: Πείρος) is a river in the central and the northwestern parts of Achaea, Greece. The river is known locally under several names including Kamenitsa (Καμενίτσα), Mellas (Μέλλας), Pieros (Πίερος), Nezeritiko (Νεζερίτικο), Prevedos (Πρέβεδος) and Acheloos (Αχελώος). One of its tributaries is the Tytheus, also known as Parapeiros.
The Peiros is the longest river in Achaea. 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 flows into the Gulf of Patras between the towns Alissos and Kato Achaia. There are several bridges over the Peiros, including the bridges of the Greek National Road 9/E55 (Patras – Pyrgos – Kyparissia) and the OSE’s railway line Patras – Pyrgos. Two ancient cities were situated near the river: Olenos and Pharae.