Planchonella sandwicensis is a shrub or tree that can grow up to 12(–20) meters tall and 60 cm in diameter. Young shoots are pale grayish, cracked, or scaly, with puberulent to short-pubescent hairs that are golden to rusty and become glabrate with age. It contains white latex. The leaves are simple, spirally arranged, and alternate. The blades are oblong, elliptic, broadly oblanceolate, obovate, or ovate, ranging from (1–)3–15(–30) cm long and (0.6–)1.2–7.5(–10) cm wide. The apex is acute to rounded, occasionally retuse, and the base is narrowly attenuate, cuneate, or acute. The upper surfaces are subglabrous, while the lower surfaces may be pubescent with whitish, golden, or rusty hairs or are glabrous. The leaf blades are thickly coriaceous to chartaceous with entire margins that are often slightly revolute. The veins are brochidodromous with a marginal vein present, and secondary veins often join to form a prominent submarginal vein. Petioles are (0.5–)1–3.5(–4) cm long, pubescent or glabrous, and not channeled. The flowers are arranged in axillary fascicles of 1–5 and are bisexual or female, with five parts (5-merous). Pedicels are (10–)12–20(–27) mm long with fawn to ferruginous tomentulose indumentum. Sepals are 3–6 mm long with round to acute apices, while the corolla is pale green, cup-shaped, 3–6.5 mm long. The stamens are inserted in the middle of the corolla tube, and the gynoecium typically has five locules, flask-shaped to subglobose. The fruit is obovoid, ellipsoid to globose, 15–33 x 10–35 mm, and ranges from purple to black (or dark red). It is crowned by a remnant style and may contain 1–5 seeds. Seeds are boat-shaped with a shiny fawn to brown testa, measuring 15–22 x 9–16 mm. The seed scar covers the adaxial surface, and the cotyledons are foliaceous and white, with an exserted radicle below the commissure. It is found in mesic forests between 500–1400 meters.