Gloom is a blue, bipedal Pokémon with rudimentary hands and feet. On its head is a flower with spherical petals surrounded by four reddish-brown leaves. The petals are a dark reddish-brown and have white spots. A female displays a single large spot on each petal, while a male will have many smaller spots. Gloom's eyes are wide and often appear closed and its lips are thin and purple.

Gloom trickles sticky nectar from its mouth with the appearance and consistency of drool. This can be used as a fertilizer for plants. Both this nectar and the flower on Gloom's head release a foul odor strong enough to induce fainting in humans over a mile (1.6 kilometers) away.