The mountain serpent eagle (Spilornis kinabaluensis) is a fierce-looking, forest-dwelling eagle found only on the island of Borneo. A relatively small eagle, it has dark brown plumage that is paler and speckled on the underparts. The flight feathers of the long wings have black tips and white bases. The head and throat of the mountain serpent eagle are black with light speckles on the back of the neck, and it has a short, bushy crest of feathers on top of the head.

The long, dark tail of the mountain serpent eagle has a thick white band running across it, a feature that differentiates it from its close relative the crested serpent eagle (Spilornis cheela), which has a less distinctive greyish-white tail band. The mountain serpent eagle also has larger wings, smaller speckles and is generally darker in appearance than the crested serpent eagle.

Like all birds of prey, the mountain serpent eagle has an incredibly sharp, hooked beak, used for tearing apart its prey. The talons and beak are bright yellow and stand out against the dark body.

The call of the mountain serpent eagle is a repeated, high-pitched whistle.