Euphorbia arnottiana is a shrub with erect to spreading stems that grow between 0.5 and 1.5 meters, occasionally reaching 2 meters long. The branches are slender, flexuous, and glabrous. Its leaves are simple, opposite, and distichous, with ovate to broadly lanceolate blades measuring 17–50 mm long and 10–30 mm wide. The leaf apex is acute to acuminate, sometimes retuse, while the base is rounded to obtuse and slightly oblique. The surfaces are glabrous, with serrate margins. Petioles are 1.5–4 mm long and pilose above, while the stipules are triangular, 0.5–1 mm long, erose, and ciliate. The flowers are in cyathia, arranged in branched cymose inflorescences, or sometimes solitary in the leaf axils. Peduncles are 1–2(–3) mm long and glabrous. The involucre is campanulate, measuring 1.5–2(–2.5) mm high and 1.5–2 mm wide, with glabrous to pilose surfaces beneath the four green glands. Flowers are unisexual and highly reduced, with pistillate (female) flowers occurring solitarily on a gynophore, while staminate (male) flowers are arranged in five groups or cymules, each containing one to several flowers. Both the calyx and corolla are absent. The male flowers are reduced to a single stamen, with an articulation at the junction of the pedicel and filament, subtended by slender bracteoles. The ovary is superior, with white to pink styles that are bifid nearly half their length (0.3–0.5 mm) and thickened stigmas. The fruit is a green capsule, 2–3 mm long, glabrous to villous along the sutures, becoming glabrate at maturity. Gynophores are recurved and exserted 2–3 mm from the involucre, pilose. The white to brown seeds are 1.5–1.7 mm long and transversely rugose. It is typically found in dry shrublands and mesic forests between 300-600 meters.