Nakai-Nam Theun National Park in Nakai District, Khammouane Province, Laos, is one of the last remaining wildernesses in Southeast Asia.

Nakai-Nam Theun covers approximately 4,270 km2 of the Annamite Range and the adjacent Nakai Plateau in Khammouane and Bolikhamsai Provinces.

From north to south, riversheds in the park consist the following rivers:[3] All are tributaries of the Nam Theun to the southwest in the Nakai Plateau.

Wet evergreen upper montane fagaceous forests occur above 1,800 meters elevation, and include species of oak (Quercus), Castanopsis, and Lithocarpus.

Birds present in the park's upper montane forests include the threatened beautiful nuthatch (Sitta formosa), along with species characteristic of the wet evergreen rain forests on the eastern slope of the Annamites, like the Vietnamese crested argus (Rheinardia ocellata) and short-tailed scimitar babbler (Napothera danjoui).