Rillaboom is a bipedal, ape-like Pokémon. Its body is mostly mud-brown in coloration, including its face and body, as well as its thick arms and legs. Its face has an orange snout, with a dark brown nose. This dark brown coloration is also seen on its triangular ears, as well as under its eyes, which themselves have orange irises and white pupils. Its forehead is a light tan color, as well as its chest, abs, hands, and feet. It also has a bow-shaped mark on each shoulder of the same color. Rillaboom also has leaves protruding from its head, creating a thick bush of hair on its head, with a single vine that falls from its forehead like a strand of hair. It also has a belt and cuffs made of leaves. Rillaboom always carries its signature drum with it on its back, and pulls it out when engaged in battle. This bass drum is crafted from a tree stump, with several thick branch stumps protruding from it, which form both the legs of the bass drum as well as mid and high toms.
By drumming, Rillaboom can tap into the power of its special tree stump. The roots of the stump follow the direction of drumming in battle. A Rillaboom with the best drumming technique can become the boss of its troop. Due to its kind disposition, it values harmony with the group. Rillaboom is very sensitive towards discord that could threaten the group's harmony, so it keeps a strict watch for such actions. Due to its strong sense of unity, it will ferociously defend its allies from trouble. Rillaboom is the only known Pokémon capable of using the the move Drum Beating.