Alfeiós (, also romanized as
Alpheus,
Alpheios) is the longest river in the
Peloponnese, in
Greece. The river is 110 km long, flowing through the regional units of
Arcadia and
Elis. Its source is near the village
Dorizas, about halfway between
Tripoli and
Megalopoli in the highlands of
Arcadia. It flows southwest toward Megalopoli, where it has been diverted around open pit lignite mines. At
Thoknia it receives its right tributary Elissonas, and continues north towards
Karytaina. Below Karytaina the
Lousios flows into the Alfeios, and the Alfeios continues northwest, passing north of
Andritsaina. Near
Tripotamia the rivers
Ladon and
Erymanthos flow into the Alfeios. The Alfeios then flows west along
Olympia and empties into the
Ionian Sea south of
Pyrgos.