Kallanai is a dam built across the river Kaveri in 1st century BCE in southern India. The project was undertook by the ancient Chola king Karikalan. It is considered the oldest water-diversion structure in the world still in use.

The word Kallanai literally means stone dam in Tamil. The structure is 329 meters long and 60 meters wide, and as the name implies, is made of stone. The purpose of the dam is to divert the waters of river Kaveri to the delta regions for irrigation via irrigation canals.