The Osa Peninsula is home to 2.5% of the world's biodiversity and more than 50% of the biodiversity in Costa Rica. It is the largest continuous forest on the Pacific Coast in Central America. It also represents one of the last tropical rainforests with sufficient extension to support viable populations of Central American flora and fauna, being recognized as a priority area for conservation since the 1970s.

The altitudinal variation ranges from sea level to 745 meters above sea level, the climate is mainly warm and humid and annual precipitation reaches 6000 mm. These conditions, among others, make this place particularly diverse in terms of landscapes, ecosystems, and species.

Much of the territory is under some protection regime, either by the state or privately. The Corcovado National Park is the most famous but there are also the Térraba Sierpe National Wetland, the Golfo Dulce Forest Reserve, and the Caño Island Biological Reserve.

Endemism

Many of the species that occur in the Osa Peninsula are endemic, including twenty species of trees, some birds and the poisonous frog Phyllobates vittatus, a threatened species according to the IUCN.

Costa Rica's southern Pacific lowland rainforest is relatively isolated from other forested areas, including the lowland forests of the country's Atlantic region. The Talamanca Mountain Range has acted as a barrier, which has limited the dispersal of birds between the rainforests of the Pacific and Caribbean lowlands.