Achankovil is a 128 km long west flowing river in Kerala, India, which flows through Kollam, Pathanamthitta, and Alappuzha districts. The river drains vast tract of fertile plains of Upper Kuttanad in the Alappuzha and Pathanamthitta districts. It also sustains numerous urban settlements along its course such as Konni, Pathanamthitta, Pandalam, Mavelikkara etc.
The river has its origin in Devar Mala peak of the Western Ghats situated in the Konni Reserve Forest. Rivers like Rishimala, Pashukidamettu and Ramakkaltheri form the major tributaries of the Achenkovil river. It flows south west from its origin and enters Kollam district near Achencovil village. Then the river switches its course towards north west and flows almost 20 kms through Kollam district before re-entering Pathanamthitta district near Kalleli.
Later it flows through the hilly towns of Konni, Kumbazha, Mylapra, Konnithazham, Vettoor, Pathanamthitta, Pramadam, Vallikode and enters into the Upper Kuttanad. Then the river passes through the towns of Kaipattoor, Nariyapuram, Thumbamon, Venmony, Kulanada, Pandalam, Edappon (Nooranad), Vettiyar, Kollakkadavu and Cheriyanad. After reaching Mavelikkara, the river takes a sharp turn towards the northern direction and flows through the following towns of Chennithala, Pallippad, Paippad and finally merges into the Pamba river at Veeyapuram. The Pamba River together with Achankovil and Manimala rivers, then splits into two distributaries, one which empties into the Arabian Sea at Thottapally and the other into the Vembanad Lake at Kainakary.
The Pathanamthitta Town, which is the administrative capital of the Pathanamthitta district is situated along the river. The town derives its name from its association with the river; Pathanamthitta, from the Malayalam words and , means "houses by the riverside".
The forest area, which is the catchment area for this river is also referred to as Achenkovil Reserve Forest. It forms part of Agasthyamala Biosphere Reserve.
A small village situated in its upper riparian area of the river is called as Achenkovil village.