The reserve is located between latitude 8° 25' and 8° 53' N and longitude 77° 10' and 77° 35' E, about 45 km west of Tirunelveli Town, and forms the catchment area for 14 rivers and streams. Among these rivers and streams, the Ganga, Tambraparani, Ramanadi, Karayar, Servalar, Manimuthar, Pachayar, Kodaiyar, Gadananathi River, and Kallar form the backbone of the irrigation network and drinking water for the people of Tirunelveli, Turicorin and part of Kanyakumari District. Seven major dams—Karaiyar, Lower Dam, Servalar, Manimuthar, Ramanadi, Gadananathi River and Kodaiyar—owe their existence to these rivers.
The reserve spans a range of 40 to 1,800 m in elevation. Agasthiyamalai (1681 m.) is in the core zone of the reserve.