Gardenia mannii is a tree that grows 5–15 meters tall. The branches are somewhat quadrangular, viscid, and appressed puberulent. The leaves are simple and opposite, clustered toward the tips of the branches, and can be thin or occasionally thick and chartaceous. The blades are oblanceolate to elliptic-oblanceolate, measuring 6–27 cm long and 3.5–10 cm wide. The upper surfaces have remnants of viscid exudate and are puberulent along the midrib. In contrast, the lower surfaces are puberulent, especially along the veins, with domatia usually present in the axils of the lateral veins. The margins are entire, with 10–18 prominent lateral veins. The petioles are 0.1–0.8 cm long and subappressed puberulent. Stipules are connate into a collar 3–7 mm long, viscid, and puberulent. The flowers are solitary, terminal, and have 3–17 mm long pedicels, 4–6-angled, viscid, and appressed puberulent. The hypanthium is narrowly funnelform, 8–12 mm long, 4–6-angled, often costate, and formed by the lower part of the calyx and enclosed ovary, with a shallow nectary disk apically lined. The flowers are insect-pollinated, bisexual (perfect) or rarely unisexual, often heterostylous, fragrant, and open in the late afternoon, lasting about two days. The calyx is cup-shaped, 3–5 mm long, viscid, and puberulent, with 4–6 terminal spurs that are coriaceous, obliquely linear-spatulate, and sigmoid, rarely only falcate, 23–46 mm long and 5–11 mm wide, sparsely puberulent. The corolla is cream-colored externally and white within, thick, and 7–9-lobed. The tube is 17–27 mm long (when dry), and the lobes are 16–22 mm long (when dry). The stamens are as many as and alternate with the corolla lobes, included, with short filaments usually inserted on the corolla tube or throat. The anthers are 14–16 mm long, dithecal, opening by longitudinal slits, dorsifixed above the middle, linear-oblong, and included or scarcely exserted from the corolla at anthesis. The ovary is inferior, occasionally partly so, or rarely superior, 1-celled or falsely 2-celled, with 2–8 parietal placentas and numerous ovules. The style is terminal and slender, with stigmas connate or 2-lobed, dry, or occasionally wet. The fruit is yellow to orange, broadly ellipsoid, 1.8–4.5 cm in diameter, prominently costate, and terminated by somewhat persistent calyx spurs. The pericarp is hard, with a thin exocarp and fibrous-coriaceous mesocarp. The endocarp is bony, and the pulp is bright reddish orange. The seeds are numerous, horizontally embedded in the pulp, straw-colored, ovoid to ellipsoid, compressed, 2.0–2.6 mm long, with well-developed oily or occasionally scanty or absent endosperm. It is found in mesic to occasionally wet forests between 100-730 meters.