Sinharaja Forest Reserve is a forest reserve and a biodiversity hotspot in Sri Lanka. It is of international significance and has been designated a Biosphere Reserve and World Heritage Site by UNESCO. Sinharaja Forest Reserve covers most of the Kalu Ganga basin and a small part of northern Gin Ganga. Most of the forest (60%) is contained within the borders of Rathnapura District. The other parts include Galle District with 20% and Kaluthara District with 20%.

This forest encompasses an area of 36,000 hectares (88,960 acres/360 km²). The reserve is only 21 km (13 mi) from east to west, and a maximum of 7 km (4.3 mi) from north to south, but it is a treasure trove of endemic species, including trees, insects, amphibians, reptiles, birds, and mammals. Sinharaja forest vegetation density has been estimated at around 240,000 plants per hectare, the most dense rain forest in Asia.

According to the International Union for Conservation of Nature (IUCN), Sinharaja is the country’s last viable area of primary tropical rainforest. More than 60% of the trees are endemic and many of them are considered rare. 50% of Sri Lankan’s endemics species of animals (especially butterfly, amphibians, birds, snakes and fish species). It is home to 95% endemic birds.