|This article does not cite any references or sources. (February 2007)|
|Length||approximately 25 km|
Larissos (Greek: Λάρισσος, also Riolitiko) is a river in the western part of Achaea, southern Greece. Its source is on the western slope of the Movri hills, near the village Mataragka. It flows through the Prokopos lagoon, and empties into the Ionian Sea in the village Kalogria. Other villages along its course are Kagkadi, Apideonas and Lappas. Its complete course lies within the municipal unit Larissos, which took its name from the river.
In Antiquity it was the boundary between Achaea and Elis. Near the river the ancient cities Dyme and (according to Strabo) Larissa were located. According to Greek mythology, Heracles drank the waters on his way to clean the Aegean stables.