Prostanthera spinosa is a small, rigid, upright shrub with a scrambling habit mostly semi-prostrate, usually 0.2–2 m (7.9 in – 6 ft 6.7 in) high.

The leaves are darker on the upper surface, paler on the underside, smooth or with firm, spreading hairs below on the midrib and profusely covered with glands.

The leaf margins entire or slightly rolled under, underside veins obscure, ending in a rounded apex on a petiole 0.4–1 mm (0.016–0.039 in) long.

Its flowers occur singly in the leaf axils, they are pale mauve to lilac to white with orange-brown streaks or spots on the lower inside petal that is 2–3 mm (0.079–0.118 in) long.

[3][4][5] The species was formally described in 1855 by Victorian Government Botanist Ferdinand von Mueller, based on plant material collected from "rocky declivities in springs near the Grampians".