The Achankovil Aar is a river in Kerala, India, formed towards the southern tip of the peninsula from the streams of the Rishimala River, Pasukidamettu River, and the Ramakkalteri River. This river enriches the Pathanamthitta district of Kerala state. It joins with the Pamba River at Veeyapuram, in the Alappuzha district of Kerala in South India. Achankovil is also the name of the forest area, which is the catchment area for this river, and of a small town situated in the Achenkovil forest area. The Achankovil village is not easily accessible; however, it can be reached through forest routes.

The River Achankovil nurtures numerous townships on its fertile banks. Among them is Pathanamthitta Town, which is the capital of the Pathanamthitta district. It derives its name from its association with the river; Pathanamthitta, from the Malayalam words Pathanam and thitta, means "houses by the riverside".

Other townships/ places include:

The river is unique in that along its route is a large number of ancient temples, indicating that the richness of the river basin has been identified by humans since ancient times and they preferred to settle down there so that they could grow their crops on the fertile lands. Edappon, a place famous for its flora and fauna, is on the banks of the Achankovil river.