Melicope haupuensis is a tree reaching approximately 8 meters in height, characterized by new growth that is appressed puberulent. The leaves are simple and opposite, with ovate to elliptic-ovate blades measuring 5–13 cm long and 2.8–5.6 cm in width. The apex of the leaf is rounded to subacute, and the base is rounded or subcordate. The surfaces of the leaves are glabrous and chartaceous, featuring entire margins. An arched submarginal vein connects the pinnate veins, and the higher-order venation forms a reticulate pattern. The petioles are relatively long, ranging from 20 to 50 mm, and stipules are absent. The flowers appear in axillary cymes, numbering 5–7, and exhibit sparse appressed puberulence. The peduncles measure 2–7(–10) mm long, and the pedicels are 1–3 mm long. The flowers can be either bisexual (perfect) or functionally unisexual, with the plants possibly being monoecious. The calyx consists of 4 sepals, which in pistillate flowers are deltate, about 2 mm long, and densely appressed puberulent. The corolla comprises four valvate petals, with those of pistillate flowers being oblong-deltate and about 3.5 mm long, also puberulent. The stamens are organized into 8, arranged in 2 whorls, and are distinct; they are reduced but always present in functionally pistillate flowers. The fruit comprises distinct follicles measuring 18–22 mm in width and 9–11 mm in length, with both the exocarp and endocarp being glabrous. The carpels dehisce along the upper suture. Each fruit cell contains 1–2 glossy black seeds when ripe, crustaceous, and ovoid, sometimes angled due to compression. The embryo is straight and embedded in fleshy endosperm, with the radicle short and the cotyledons compressed and ovate. The putamen is brown and warty. It is found in mixed mesic forests, on ridges, slopes, and near streams at an elevation of approximately 375 meters.