The Alfeios (Greek: Αλφειός, also Alfios) is a river in Peloponnese, Greece. Its source is near Megalopolis in the prefecture Arcadia. It flows along Olympia and empties into the Ionian Sea in Elis prefecture, near Pyrgos. The river flows in the prefectures of Arcadia and Elis and is the longest river in the Peloponnese and the length is 110 km.
A poem called Le fleuve Alphée (the Alpheus River) by Roger Caillois is mainly about this river.
The river begins near Davia in central Arcadia, then it flows between Leontari and Megalopoli and the municipal boundary of Falaisia and Megalopoli in a wooded valley and south of Karytaina and flows north of Andritsina and for about 15 to 20 km with the prefectural boundary with Ilia and Arcadia. It later flows with the provincial bountary of Olympia and Ilia for the rest of the length and the municipal boundary of Olympia and Alifeira to the south, it later flows to the municipal boundary of Skillounta to the south and later with the Pyrgos and Volaka boundary and finally for 2 km, into the wetlands with bushes.