Gardenia cornuta is a shrub or multi-branched small tree, 3 to 5 m tall, with erect and spreading main branches, branchlets sometimes with thick thorns, a trunk that is whitish grey, the bark is smooth, often flaking into thin peels.

Leaves are smooth and crowded at the ends of twigs, usually in whorls of three, light glossy green, short-stalked, 20-30 mm long and broad, margins smooth; stipules joined to form a ring around the node.

Flowers are solitary at the ends of short, rigid branches, borne in summer, November to March, white turning yellow, short-stalked, with a long, slender greenish tube, 50-60 mm, and spreading white lobes, 20-30 mm long. The calyx is very unusual, with 6 long, narrow lobes arising from its sides and not at its apex. Fruits which are more attractive than the flowers, are more or less pear-shaped, woody, tipped with persistent remains of the calyx, glossy golden yellow, 40-50 x 30-35 mm, usually produced in large quantities between February and August. Seeds are hard and flattened.