The Lousios (Greek: Λούσιος; Latin: Lusius), also known in antiquity as Gortynius or Gortynios (Ancient Greek: Γορτύνιος),[1] is a river and a gorge in western Arcadia that stretches from Karytaina north to Dimitsana in Greece.

It empties into the Alfeios 2.5 kilometres (1.6 mi) northwest of Karytaina and south of Atsicholos.

At the southern end is the ancient city of Gortys with the temple of Asclepius.

According to tradition, the river took its name from Zeus, the father of the Olympian gods, who according to Pausanias washed at its sources after his birth.

[2] The Lousios gorge is also known as the "Mount Athos of the Peloponnese" on account of the many monasteries that dot its walls: the Timiou Prodromou, Philosophou, Aimyalon, Panagias Kalamiou monasteries, and the Church of St. Andrew in Gortys.