Euphorbia halemanui is a scandent shrub ranging from 1 to 4 meters with glabrous stems and flowering branches measuring 2 to 3.5 mm in diameter. The leaves, set in opposite, decussate pairs, feature elliptic to oblanceolate blades measuring 40–130 mm in length and 10–45 mm in width, with a broadly acute to rounded apex, occasionally showcasing retuse or mucronulate tips, and a cuneate and oblique base. The leaf surfaces are glabrous or slightly pubescent at the basal margin, circumscribing entirely smooth margins and connecting to petioles 3–11(–16) mm long, either glabrous or pilose above. Stipules are persistent, acute–triangular, measuring 1–3 mm long, and ciliate. The flowers, unisexual and highly reduced, present in cyathia within dense, glomerate, branched cymose clusters or sometimes as solitary entities, with peduncles 3–6 mm long when cyathia are solitary or 1–3 mm long when in cymes. Involucres are narrowly campanulate, measuring 1.5–2 mm in height and width, glabrous, with 4–5 yellow to green glabrous glands. The pistillate flowers stand alone on a gynophore, while the staminate flowers group in 5 cymules, each bearing one to several flowers. Calyx (sepals) and corolla (petals) are absent, with male flowers reduced to a single stamen. Fruits are green capsules, 2–3 mm long, glabrous, with gray to brown seeds, approximately 2 mm long, and shallowly rugose. This species thrives in diverse mesic forests and occasionally in wet forests at elevations ranging from 400-1,100 meters.