Larissos (, 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.