The Asopos (, referred to in Latin sources as Asopus) is a river in Boeotia and northern Attica, Greece. In antiquity, it formed the border between the cities of Thebes and Plataea. The Battle of Plataea was fought on its banks. According to Pausanias (5.14.3) the Boeotian Asopus can produce the tallest reeds of any river.