Fonissa (Greek: Φόνισσα) is a stream in western Corinthia, Peloponnese, Greece. It is 13.3 km (8.3 mi) long and flows into the Gulf of Corinth. The source of the river is in the hills near the village . It flows towards the southeast though a narrow gorge, and empties into the Gulf of Corinth between the villages and , 6 km northwest of Xylokastro.