Gunung Leuser National Park represents several ecosystem types, from coastal forest ecosystem through tropical lowland forest ecosystem to montane forest ecosystem. Most of the Park area is covered with thick Dipterocarpaceae forest with rivers and waterfalls flowing through it. There are some endangered and peculiar plants, namely daun payung raksasa (Johannesteijsmannia altifrons), rafflesia flowers (Rafflesia atjehensis and R. micropylora), and Rhizanthes zippelnii-the biggest flower, with a diameter of 1.5 metres. In addition, there is one plant unique to the area: the ara, a strangling plant.
Endangered and protected animal species which inhabit the Park include orangutan (Pongo abelii), siamang (Hylobates syndactylus syndactylus), Asian elephant (Elephas maximus sumatranus), Sumatran tiger (Panthera tigris sumatrae), Sumatran rhinoceros (Dicerorhinus sumatrensis sumatrensis), serow (Capricornis sumatraensis), great hornbill (Buceros bicornis), sambar deer (Cervus unicolor), and leopard cat (Prionailurus bengalensis sumatrana).
Gunung Leuser National Park has been declared a Biosphere Reserve. Under a cooperation program between Indonesia and Malaysia, the Park is also designated as a "Sister Park" to the Taman Negara in Malaysia.