Sinharaja Forest Reserve is situated in the south-west lowland wet zone of Sri Lanka. Most of the area was originally declared a forest reserve on 1875 under the Waste Lands Ordinance and notified in the Ceylon Government, whereas the rest was notified a proposed forest reserve in the early 20th century. Sinharaja Forest Reserve, comprising the existing and proposed forest reserves, was declared a biosphere reserve in 1978.

This narrow strip of undulating terrain consists of a series of ridges and valleys. Two main types of forest can be recognized: remnants of Dipterocarpus forest occur in valleys and on their lower slopes; secondary forest and scrub occur where the original forest cover has been removed by shifting cultivation and in other places the forest has been replaced by rubber and tea plantations. Mesua-Doona forest is the climax vegetation in most of the reserve. Of Sri Lanka's 830 endemic species, 217 trees and woody climbers are found in the lowland wet zone. Other rare endemics are the palm, the latter being restricted to Sinhagala. A variety of plants of known benefit to man are present, of which palm (for jaggery, a sugar substitute), wewal, cardamom, dun (for varnish and incense) and weniwal (for medicinal purposes) are used intensively by villagers.