Melicope quadrangularis is a shrub or small tree with new growth that is yellowish-brown and appressed-puberulent, becoming glabrate as it matures. Leaves are simple and opposite, with elliptic to elliptic-obovate blades measuring 9.5–16 cm long and 4.5–7.5 cm wide. The apex is rounded to subacute, and the base is cuneate. Upper surfaces become glabrate, while lower surfaces are sparsely pilose, especially along the veins. The blades are thin and coriaceous, with entire margins. Primary lateral veins number 18–20 pairs and are connected by an arched vein 1–2 mm from the margin; venation is pinnate, with higher-order veins forming a reticulate pattern. Petioles are 10–12 mm long, and stipules are absent. Flowers occur 1–2 per glabrous axillary cyme, with peduncles 0–30 mm long and pedicels 20–35 mm long; bracteoles are sparsely puberulent, and bracts are small and opposite. The calyx has 4 sepals, and the corolla has 4 petals, both white or white tinged pink or purple. There are 8 stamens. The ovary is superior, 4-celled and 4-lobed, with carpels connate and only slightly distinct at the apex to almost entirely distinct; ovules are 2 per cell, hemitropous, with 1 ascending; the single style terminates in 4 short stigmatic branches. Fruits are depressed-cuboid capsules, 12–14 mm long and 19–22 mm wide, with carpels completely connate and a conspicuous central depression at the apex. The exocarp is very sparsely puberulent, the endocarp is glabrous, and the carpels dehisce along the upper suture. Seeds are 1–2 per cell, glossy black when ripe, crustaceous, ovoid, sometimes angled from compression, with a straight embryo in fleshy endosperm, short radicle, compressed ovate cotyledons, and a brown, warty putamen. It occurs in montane habitats at approximately 920 meters.