Box elder is a medium-sized tree, 40–70 feet tall, with a broad, uneven crown.
Leaves opposite, pinnately compound with 3–5 leaflets arranged like a feather. Leaflets are 2–4 inches long, 1½–3 inches wide, broadest near the base, with margin coarsely toothed, sometimes with short lobes, upper surface light green, lower surface grayish green.
Bark smooth and green when young; when older, pale gray to brown and separating into long thin ridges with shallow grooves.
Twigs slender, smooth, shiny or with a whitish coating that rubs off easily, green to olive green, sometimes purple or brownish.
Flowers April–May, with male and female flowers on separate trees. Flowers small, greenish, drooping, on slender stalks, petals absent.
Fruits August–October, often remaining on the tree over winter; in drooping clusters 6–8 inches long. Fruits are winged, paired, 1–2 inches long, similar to maple seeds.