Pisonia brunoniana is a small tree, spreading to 6 metres (20 ft) or more tall. The wood is soft and the branches are brittle. The large leaves are opposite or ternate, glabrous, and glossy, entire (simple with smooth margins), and obtuse to rounded at apex. The inflorescence is paniculate, many-flowered, and the flowers are unisexual. The very sticky fruits, in which small birds are often trapped, are narrowly ellipsoidal, and 2–3 centimetres (0.79–1.18 in) long, having five ribs. In the Seychelles, the sticky seeds of a related species, Pisonia grandis, regularly causes seabird deaths, and research suggests that the seeds evolved to be transferred on the plumage of seabirds to distant islands, enabling the long-distance dispersal of the species. The situation may be similar with P. brunoniana.