In Periyar Tiger Reserve there is a rich diversity of vertebrates. The diverse forest types, vayals, marshes, and a large aquatic habitat together support 62 species of mammals, 320 species of birds, 45 species of reptiles, 27 species of amphibians, and 38 species of fishes.
The invertebrate fauna of Periyar is not well documented, though the predominant invertebrate orders are Protozoa, Annelida, Arthropoda and Mollusca. 160 butterfly species have been listed.

Mammals
Among the 62 species of mammals, many are rare, endemic and endangered. Periyar is prime elephant country and large herbs are often sighted at the lake fringes. There is an estimated population of 900-1000 animals which is likely to overlap with the population in the adjoining forest areas. A much disproportionate sex ratio of the elephant is found in Periyar and adult tuskers are far too few. Gaur, among the largest of bovines, occurs in all types of habitats. Gaur have staged a remarkable comeback after their near demise due to a rinderpest outbreak in the 70’s.
Sambar, the largest deer in India are extensively distributed, and constitute the principal prey base of tiger and wild dog. Wild pig, has wide distribution throughout the park. Some times these animals invade agricultural lands often leading to man-wildlife conflicts. Small relict populations of the highly endangered Nilgiri tahr are found in the high altitude grasslands of Mangaladevi. Four of the five primate species found in the Western Ghats are well represented in Periyar. While the Nilgiri langur enjoys a wide distribution in the moist forests, the highly endangered Lion tailed macaque are confined to the dense evergreen canopies. Malabar giant squirrel is common in the area. Though the presence of the endangered species, Small travancore flying squirrel is recorded from Periyar, the Large flying squirrel is more frequently seen.

Salim Ali’s fruit bat reported from Periyar adjoining the High Ways. is an endemic and endangered species, considered to be among the rarest of bats

Tiger
The Tiger is found in all types of habitats though their density is much less in the evergreen forests. Though sighting one is difficult because of the thick vegetation, evidences like pugmarks, clawings, and scats are located throughout the reserve. The population is estimated to be between 35–40.

The Striped necked mongoose is not uncommon, found in the semi-evergreen and evergreen forests.

Nilgiri marten is an extremely rare and endemic mustellid, though recently sighted a number of times.