The Periyar India protected area lies in the middle of a mountainous area of the Cardamom Hills. In the north and the east it is bounded by mountain ridges of over 1700 metres altitude and toward the west it expands into a 1200 Meter high plateau. From this level the altitude drops steeply to the deepest point of the reserve, the 100 Meter valley of the Pamba River. The highest peak is the 2019 Meter high Kottamalai.
The sanctuary surrounds picturesque 26 km² Periyar lake, formed by the building of Mullaperiyar Dam in 1895. This reservoir and the Periyar River meander around the contours of the wooded hills, providing a permanent source of water for the local wildlife.
The temperatures vary depending upon the altitude and it ranges between 15° Celsius in December and January and 31° Celsius in April and May. The annual amount of precipitation lies between 2000 and 3000 mm. About two thirds of the precipitation occurs during the southwest monsoon between June to September. A smaller amount of precipitation occurs during the northeast monsoon between October and December.
Approximately 75% of the entire protected area are covered of unaffected evergreen or semi-evergreen rain forest. There typically tall tropical tree species such as Vateria indica, Cullenia exarillata, Hopea parviflora, Canarium strictum, Artocarpus hirsutus and Bischofia javanica are seen. They reach heights of 40 to 50 Metres.
Scarcely 13% consists of damp leaves forest, 7% of Eucalyptus plantation and 1.5% of grassland. The remainder (around 3.5%) of the protected area is covered by the Periyar artificial lake as well as the Periyar River and Pamba rivers.
Altogether the reservation counts nearly 2000 kinds of flowering plants (Angiosperms), three kinds of seed plants (gymnosperms) and 170 different species of ferns. Among the Angiosperms, there are 169 families of sweet grasses and 155 kinds of Fabaceae. Orchids, with 145 representative types, are the most frequent flower. About 350 of the occurring plant types can be used for medical purposes.