Eucalyptus polyanthemos, commonly known as red box, is a species of small to medium-sized tree, that is native to eastern Australia but has been introduced into other countries. It has fibrous bark on the trunk and larger branches, smooth greyish to cream-coloured bark above, or smooth bark throughout. It has broadly egg-shaped to round juvenile leaves, lance-shaped, egg-shaped or almost round adult leaves, flower buds in groups of seven, white flowers and barrel-shaped to conical fruit.
Eucalyptus polyanthemos is a tree that typically grows to a height of 20 m (66 ft) but does not form a lignotuber. It has fibrous or flaky bark on the trunk and larger branches, smooth mottled greyish, cream-coloured and yellow bark above, or sometimes smooth bark throughout. It often has a crooked trunk and is noted for its domed canopy of greyish foliage. Leaves on young plants are green to bluish grey, broadly egg-shaped to more or less round, 25–80 mm (0.98–3.15 in) long and 25–65 mm (0.98–2.56 in) wide and petiolate. Crown leaves are the same shade of dull green to bluish or greyish on both sides, lance-shaped to egg-shaped or round, 50–110 mm (2.0–4.3 in) long and 18–50 mm (0.71–1.97 in) wide tapering to a petiole 10–27 mm (0.39–1.06 in) long. Veins on the leaves are distinct and the marginal vein is notably distant from the leaf edge.
The flower buds are arranged on the ends of branchlets in groups of seven on a branching peduncle 2–10 mm (0.079–0.394 in) long, the individual buds on pedicels 1–5 mm (0.039–0.197 in) long. Mature buds are oval to diamond-shaped, 3–5 mm (0.12–0.20 in) long and 2–3 mm (0.079–0.118 in) wide with a conical to slightly beaked operculum. Flowering occurs in October and November (spring in Australia) and the flowers are white. The fruit is a woody, barrel-shaped to conical capsule 3–6 mm (0.12–0.24 in) long and wide with the valves below the level of the rim.