Sinharaja Rainforest is home to a diverse range of flora and fauna, including several endemic species that can only be found in this region of Sri Lanka. The forest is particularly renowned for its birdlife, with over 147 species recorded within its boundaries. Many of these birds are endemic to Sri Lanka, including the Sri Lankan blue magpie, Sri Lanka junglefowl, and Layard’s parakeet. The forest is also home to many species of mammals, including several primates such as the purple-faced langur, toque macaque, and endemic Horton Plains slender loris. The elusive Sri Lankan leopard also inhabits the forest, although sightings of these big cats are rare. The streams and rivers that run through the forest are home to a variety of fish and amphibians, including the endangered Sri Lanka torrent frog. Reptiles such as the green pit viper, hump-nosed viper, and Sri Lankan krait can also be found in the forest.

Sinharaja Rainforest is renowned for its unique and diverse range of endemic plant species. The forest is home to over 60% of the tree species found in Sri Lanka, with many of them being endemic to this region. Some of the notable endemic species include the Dipterocarpus zeylanicus, Mesua ferrea, and Calophyllum walkeri. Additionally, there are several rare orchid species that can be found in the forest, making it a haven for plant lovers and botanists.