The Lousios, also known in antiquity as Gortynius or Gortynios, is a river and a gorge in western Arcadia that stretches from Karytaina north to Dimitsana in Greece. The river begins near Lykochori and flows through the Lousios Gorge. The river is treacherous and flows rapidly. It empties into the Alfeios 2.5 kilometres (1.6 mi) northwest of Karytaina and south of Atsicholos.
The river forms a deep, narrow gorge. Its length is approximately 15 km (9.3 mi) from north to south and its width is approximately 2 km (1.2 mi) wide. The gorge is very popular amongst hikers. At the northern end of the gorge lies the town of Dimitsana. At the southern end is the ancient city of Gortys with the temple of Asclepius. Much of the gorge is heavily forested, and there are steep cliffs.
