Euphorbia atrococca is a shrub or small tree with brown bark that grows between 1 and 3 meters tall. The stems are freely branched above and glabrous, with numerous secondary branches measuring 1–1.5 mm in diameter. The leaves are simple, opposite, and decussate. The blades are narrowly elliptic to elliptic, oblanceolate, or obovate, measuring 20–60 mm long and 5–21 mm wide. The apex is acute to rounded and rarely mucronulate or retuse. The base is cuneate and oblique, with glabrous surfaces and entire margins. The petioles are 2–6 mm long and glabrous. Stipules are rounded–triangular, 1–2 mm long, and ciliate. The flowers occur in cyathia, which are either solitary in the leaf axils or arranged in cymes of 1–3 flowers on slender, contracted branches that are usually clustered at the ends of the main branches. Peduncles are 1–2 mm long and glabrous. The involucre is campanulate, 2–2.5(–3) mm high and (2–)2.5–4 mm wide, glabrous to villous beneath the glands. Each cyathium has 4 green to purple glands. The flowers are unisexual and highly reduced, with the pistillate flowers being solitary on a gynophore, while the staminate flowers are arranged in 5 groups or cymules, each with 1 to several flowers. The calyx (sepals) and corolla (petals) are absent. Male flowers consist of a single stamen, articulated at the junction of the pedicel and filament, and subtended by slender bracteoles. The ovary is superior, with bifid styles nearly half their length (0.5–1 mm long) with thickened stigmas. The fruit is a capsule, green to purple, ovoid, and keeled, measuring 3–3.5 mm long. It is villous, sometimes sparsely so, or rarely glabrous. The gynophores are recurved and exserted 1.5–3 mm from the involucre. The seeds are brown, (1.8–)2–2.3 mm long, and rugose. It occurs in dry to diverse mesic forests between 400-900 meters.