Euphorbia deppeana is a subshrub with stems that are erect to decumbent, 0.1–1.2 m long, pilose to villous, becoming glabrate as it matures; flowering branches are 1–1.5 mm in diameter. Leaves are simple, opposite, and distichous, with ovate to elliptic or rarely orbicular blades measuring 6–20 mm long and 5–12 mm wide. The apex is rounded to obtuse and often retuse, while the base is obtuse and oblique. Leaf surfaces are glabrous, and margins are serrate or rarely entire. Petioles are 0.5–1.5 mm long, glabrous or pilose above. Stipules are rounded-triangular, 0.5–1 mm long, pilose, and erose. Flowers are arranged in solitary axillary cyathia on glabrous peduncles 0.5–1.5 mm long. The involucre is campanulate, 1–1.5 mm high and 1.5–3 mm wide, glabrous below and pilose beneath the glands, with 4–5 pilose glands. Flowers are unisexual and highly reduced; pistillate flowers are solitary on a gynophore, while staminate flowers occur in 5 cymules, each with 1 to several flowers. Sepals and petals are absent. Male flowers are reduced to a single stamen subtended by slender bracteoles. The ovary is superior, with styles bifid for more than half their length (0.3–0.6 mm), and a slightly thickened stigma. Fruits are glabrous capsules 1.5–2 mm long on slightly recurved, pubescent gynophores exserted 1–1.5 mm from the involucre. It occurs on steep, exposed, windswept slopes at approximately 300 meters.