Periyar is a protected area, and a Project Tiger nature reserve in the South Indian State of Kerala, set high in the mountains of the Western Ghats at the border to Tamil Nadu. It lies in the districts of Idukki and Pathanamthitta. The protected area covers an area of 777 km², out of which a 350 km² part of the core zone was made into the Periyar National Park and Tiger Reserve, sometimes dubbed the Periyar Wildlife Sanctuary. The park is often called by the name thekkady also. Thekkady is located four km from Kumaly, approximately 100 km east of Alappuzha, 110 km west of Madurai and 120 km southeast from Kochi. The Periyar source is in the Western Ghats range in the Kerala state.Periyar is actually one of the few major rivers in Kerala which flow completely within the state. It flows north through Periyar National Park into Periyar Lake, an artificial reservoir created in 1895 by the construction of a dam across the river. The lake has an area of 55 km². Water is diverted from the lake into the Vaigai River of Tamil Nadu via a tunnel through the Western Ghats.

From the lake the river flows northwest through the village of Neeleswaram into Vembanad Lake and on the Arabian Sea coast. Its largest tributaries are the Muthirapuzha River, Mullayar River, Cheruthoni River, Perinjankutti River and the Edamala River.