The Larissos (Greek: Λάρισσος, also Riolitiko, Latin: Larisus[1]) 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 of Mataragka.

It flows through the Prokopos lagoon, and empties into the Ionian Sea in the village of Kalogria.

Its entire course lies within the municipal unit Larissos, which took its name from the river.

[1] Media related to Canal in Strofilia at Wikimedia Commons