Melicope sandwicensis is a small tree or shrub, typically reaching 3–10 meters, with new growth featuring densely minutely puberulent and tightly appressed grayish hairs. The leaves are simple and opposite, narrowly elliptic to elliptic–ovate or elliptic–obovate blades measuring 4–15(–21) cm long and 2–5(–7.5) cm in width. The apex is rounded to subtruncate and occasionally emarginate, while the base is rounded to subtruncate. The upper surfaces are glabrous except for a somewhat puberulent midrib, and the lower surfaces range from sparsely to moderately puberulent, particularly along the midrib, with pilosity; the blades are subcoriaceous to chartaceous. Margins are entire, with primary lateral veins usually forming 9–12 pairs, conspicuous, spreading-arcuate, connected by an arched vein 1–4 mm from the margin. The venation is pinnate, with lateral veins joined by an arched submarginal vein and higher-order venation forming a reticulate pattern. Petioles are 12–40 mm long, and stipules are absent. Flowering involves 1–3(–6) flowers in sparsely to densely puberulent, axillary cymes with peduncles (3–) 8–17 mm long and pedicels 3–8 mm long. Bracteoles are minute and caducous, with small, opposite bracts. The calyx comprises four oblong-ovate sepals, 2.5–3.5 mm long, densely puberulent externally and glabrous within, with ciliate margins and an acute apex that remains somewhat persistent in fruit. The corolla includes four oblong–deltate petals, each 5–6 mm long, densely puberulent with an obtuse apex and valvate arrangement. It has eight stamens arranged in two whorls, distinct and reduced but always present in functionally pistillate flowers. The superior ovary is hirtellous, leading to a style approximately 0.4 mm long and clavate stigmas not widely spreading. The ovary is 4-celled and lobed, with connate carpels and slightly distinct at the apex; each cell contains two hemitropous, ascending ovules. The style terminates in 4 short stigmatic branches. The fruit is likely dark green and measures 14–25(–30) mm wide, consisting of distinct follicles 7–15 mm long, with an exocarp that is dense to moderately grayish or pale yellowish short–pilose to puberulent and an endocarp that is sparse to moderately short–villous with short, erect or appressed, pale hairs. The carpels dehisce along the upper suture. Seeds, 1–2 per follicle and 4–6 mm long, are glossy black when ripe, crustaceous, and ovoid, occasionally angled from compression. The embryo is straight in the fleshy endosperm, with a short radicle and compressed, ovate cotyledons; the putamen is brown and warty. It grows in diverse mesic forests and occurs between 450 – 850 meters.