The Lousios (Greek: Λούσιος) 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 km northwest of Karytaina and south of Atsicholos.
The river forms a deep, narrow gorge. Its length is approximately 15 km from north to south and its width is approximately 2 km wide. The gorge is very popular amongst hikers. At the northern end of the gorge lies the town of Dimitsana. At the southern end of the gorge is the ancient city of Gortys with the temple of Asclepius. Much of the gorge is heavily forested, and there are steep cliffs. During rainy days, the gorge is dangerous to cross and is not permitted.
- Dimitsana, east of the river
- Stemnitsa, east of the river
- Gortyna, west of the river
- Atsicholos, west of the river
- Elliniko, east of the river