Euphorbia herbstii is a small tree reaching 3–8 meters in height, characterized by its diffuse branching in the upper parts and a glabrous exterior. The leaves are simple, opposite, and distichous, with narrowly oblong blades that can sometimes be narrowly oblong–oblanceolate or narrowly oblong-elliptic, measuring 6 to 19.5 cm long and 1.8 to 3.8 cm in width. The leaf apex is bluntly acute to broadly obtuse, often minutely emarginate, while the base is obliquely rounded or obliquely cuneate. The leaf surfaces are either glabrous or puberulent at the base, and the thin, coriaceous blades have entire margins. The petioles are 1.5 to 5 mm long, either sparsely puberulent or glabrous, and the plant exhibits deltate stipules that are 1.5 to 3 mm long and erose. The flowers are in a cyathia in branched, open, cymose inflorescences spanning 7 to 17 cm, with 8 to 20 mm long peduncles and puberulent. The involucre is broadly campanulate, 1.5 to 2.5 mm high, and 3.5 to 5 mm wide, featuring 5 to 6 yellowish-green glands. The unisexual flowers are highly reduced, with solitary pistillate flowers on a gynophore and staminate flowers grouped in 5 cymules, each comprising one to several flowers. Neither calyx (sepals) nor corolla (petals) are present in the flowers. The male flowers consist solely of a stamen, joined to the pedicel and filament. The fruit is a green capsule, sometimes with a reddish-purple tinge, especially on the angles, measuring 5 to 10 mm in length and up to 8 mm in diameter. The capsules are barely exserted from the involucre, with erect green gynophores that protrude approximately 1 mm from the involucre and are puberulent. The seeds are gray, round, approximately 2 to 3 mm long, with a stippled seed coat. It inhabits diverse mesic forests at elevations ranging from 360-760 meters.