Larissos (, also Riolitiko, ) is a river in the western part of Achaea, southern Greece. Its source is on the western slope of the mountain Movri, 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. It is long.