Euphorbia rockii is a compact shrub, sometimes a small tree that grows 0.5–2 meters tall. In protected sites, it can reach up to 4 meters. It is glabrous throughout. The leaves are simple, opposite, and distichous, with narrowly oblong to narrowly oblong-elliptic or occasionally narrowly elliptic blades measuring (6–)8–14(–17) cm in length and (1.5–)2–3.5(–4) cm in width. The leaf apex is obtuse to rounded, sometimes minutely emarginate, and the base is obliquely subcordate. The blades are coriaceous with entire margins. Petioles are 0–1(–2) mm long, and stipules are deltate, measuring 2–3 mm long and weakly erose. The flowers are unisexual and highly reduced, arranged in cyathia within 3–10(–12) branched, open to sometimes condensed, cymose inflorescences, typically 2–6(–7) cm long, with peduncles 3–10(–12) mm long and sparsely to moderately puberulent. The involucre is broadly campanulate, measuring 1.5–2.2 mm in height and 3.5–4.5 mm in width, and is puberulent. The plant has 5–6 glands that are greenish-yellow, green, or red. The male flowers are reduced to a single stamen. The fruits are brilliant red or sometimes pink-tinged capsules, globose, 14–25 mm long and nearly so in diameter, glabrous, and usually noticeably exserted from the involucre. The gynophores are red and erect, exserted 1–4 mm from the involucre and puberulent. The seeds are brown to grayish brown, globose or broadly obovoid, approximately 3–4 mm long, with a slightly wrinkled seed coat. It occurs in wet forests at elevations ranging from 600-830 meters.