Buxus macowanii is a small, very slow-growing tree up to about 7 m tall. It has a clean slender, greenish-brown stem of up to 300 mm in diameter but usually less. Shiny green leaves create the crown. The twigs are angled, slightly hairy when young and smooth when old. The leaves, are opposite and entire. They are arranged at right angles to each other and are up to 25 mm long. They are oval to widely lance-shaped, with a bluntly pointed or round apex and are narrow at the base. They are deeply green, leathery, smooth both sides, with the lateral veins scarcely visible.

The flowers bloom from late winter to spring (July to October), and male and female flowers are borne separately on the same tree. This is called a monoecious situation. The flowers are tiny and greenish. The male flowers are held in short heads in the axils of the leaves and the female are usually single or occasionally grouped. The fruit is a small greenish brown capsule, about 7 mm in diameter, crowned with 3 conspicuous horns, which are the remains of the styles. Fruits ripen from late summer to early winter (February to June). Once ripe, the fruit splits to release shiny black seeds.