The Sri Lanka lowland rain forests represents Sri Lanka's Tropical rainforests below 1,000 m (3,281 ft) in elevation in the southwestern part of the island. The year-around warm, wet climate together with thousands years of isolation from mainland India have resulted in the evolution of numerous plants and animal species that can only be found in rain forests in Sri Lanka.^[2] The thick forest canopy is made up of over 150 species of trees, some of the emergent layer reaching as high as 45 m (148 ft). The lowland rain forests accounts for 2.14 percent of Sri Lanka's land area.^[3] This ecoregion is the home of the jungle shrew, a small endemic mammal of Sri Lanka.^[4] Sri Lanka has the highest density of amphibian species worldwide.^[2] Many of these, including 250 species of tree frogs, live in these rain forests.

The lowland rain forests cover 124,340.8 ha (480.1 sq mi) in total and accounts for 2.14 percent of Sri Lanka's land area.^[3] Wet monsoon forests receive 2,500–1,800 mm (98–71 in) of annual rainfall and are situated belove 1,000 meters (3,281 ft) of altitude. Kanneliya, Viharakele, Nakiyadeniya, and Sinharaja, which is a World Heritage Site, are the forests that represent this ecoregion. Bambarabotuwa, Morapitiya Runakanda, Gilimale and Eratne are some of the other forest reserves.^[5] Even rainfall throughout the year, and invariable temperature resulted in rich biodiversity. These forests also act as an important catchment area for rivers.

Sri Lanka is a continental island, separated from the Asian continent only by shallow Palk Strait.^[4] Sri Lanka was once a part of Gondwanaland, until the Cretaceous Period. Then as a part of the Indian Plate, it detached and drifted northward. The Indian Plate collided with the Asian mainland about 55 million years later. Therefore, there are many ancient Gondwana taxonomic groups present in Sri Lanka. Sri Lanka first became separated from the mainland Indian subcontinent during the late Miocene Epoch. Due to climatic changes, an intermittent drier region emerged between the moist forests in southwest Sri Lanka and the Western Ghats in India, the closest other moist forests.^[4] Although the island has been connected with the mainland repeatedly by land bridges since the initial separation, Sri Lanka's moist forests and its wet forest-adapted biota have been identified as being ecologically isolated.

The ecoregion partially encircles the Central Massif, which reach above 2,500 m (8,202 ft) and detached Knuckles Mountain Range.^[4] These mountains are represented by their own ecoregion, Sri Lanka montane rain forests. The type of the soil of the ecoregion is red-yellow podzolic soil.^[3] The extended Southwestern monsoon in May to September brings more than 5,000 mm (196.9 in) of rainfall to the ecoregion. The temperature remains constantly between 27–30 °C (81–86 °F) throughout the year. Due to the proximity to the Indian Ocean daily temperatures are relieved by the ocean breezes. Relative humidity ranges between 75%-85%.^[3]

The vegetation of the region is determined primarily by climate, with topography and edaphic conditions contributing secondarily.^[4] Two floral communities dominate in the Sri Lankan lowland rain forests - the Dipterocarpus-dominated (Sinhalese "Hora") community and the Mesua-Shorea community (Sinhalese "Na-Doona"). The Dipterocarpus community comprises Dipterocarpus zeylanicus, Dipterocarpus hispidus, Vitex altissima, Chaetocarpus castanocarpus, Dillenia retusa, Dillenia triquetra, Myristica dactyloides, and Semecarpus gardneri. The Mesua-Shorea community comprises Anisophyllea cinnamomoides, Cullenia rosayroana, Mesua ferrea (the national tree of Sri Lanka), Mesua nagassarium, Myristica dactyloides, Palaquium petiolare, Shorea affinis, Shorea congestiflora, Shorea disticha, Shorea megistophylla, Shorea trapezifolia, Shorea worthingtoni, Syzygium rubicundum, and a sub canopy of Chaetocarpus castanocarpus, Garcinia hermonii, Syzygium neesianum, and Xylopia championi. Virgin forests of this ecoregion have four strata, a main canopy at 30–40 m (98–131 ft), a sub-canopy at 15–30 m (49–98 ft), a 5–15 m (16–49 ft) understory, and a sparse shrub layer.^[4] Trees of the emergent layer reach above the main canopy to 45 m (148 ft).

Swamp forests situated closer to the coastlines are a distinct habit area within the ecoregion. Avicennia-Rhizophora-Sonneratia dominated mangroves fringe the coastlines.

Almost all the endemic flora and fauna of Sri Lanka are confined to the southwestern rain forests.^[4] Due to the warm and moist climate and long physical isolation, wet forest adopted species have promoted a high degree of endemism and specialization. More than 60 percent of 306 endemic tree species of Sri Lanka are restricted to this ecoregion. A further 61 species are shared with the montane rain forests and dry forests. The dominant tree family in Asian rain forests, the family Dipterocarpaceae shows a special endemicity.^[4] All but one species of the 58 species of the family Dipterocarpaceae can only be found in these rain forests, including two endemic genera, Shorea and Stemonoporus. Anoectochilus setaceus or Wanaraja (Sinhalese for "King of forest"), an endemic orchid, is only found in undisturbed forests of this ecoregion. Several plant species show highly localized distribution. The lowland and sub montane forests are the floristically richest in Sri Lanka and of all South Asia.

Most of the Sri Lanka's rain forests were cleared for plantations, originally for coffee and cinchona and then tea and rubber.^[8] The remaining forests cover only 4.6 percent of the wet zone. Between 1990 and 2005, Sri Lanka has showed one of the highest deforestation rates of primary forests in the world.^[9][10] A survey carried out in 2005 found that 17 of Sri Lanka's frogs have become extinct in the past decade and another 11 species face imminent threat of extinction unless their habitat is secured.^[10] These remaining forests exist as highly fragmented patches.^[8] Most of them are less than 10 km² (3.86 sq mi) in extent. Poaching and the extraction of forest products (timber, firewood, medicinal plants) are a problem in almost all forest reserves.

Nevertheless, if the existing forests are preserved the condition can be improved, as many of the species of this ecoregion have small habitat needs.^[4] There are several protected areas that overlap with the ecoregion. The two most notable are the world heritage site the Sinharaja Forest Reserve and the Peak Wilderness Sanctuary. Altogether, protected areas of this ecoregion accounts for only 260 square kilometres (100.39 sq mi).

Protected areas that overlap with the ecoregion: