Barleria macrostegia is a trailing perennial herb, with much-branched, prostrate stems, growing up to 500 mm long. The branches are hairy and develop out of a woody rootstock. The leaves are opposite, oblong or elliptic and the leaf margins are wavy with long white hairs, the leaves tapering at the base and the apex (leaf tip), ending in a sharp point.

The mauve flowers are borne in a 2–4, unilateral (one-sided) flower cluster, 20–75 mm long. The corolla is 2-lipped, the upper lip 4-lobed, and lower lip 1-lobed. The petals are the same size and evenly spaced. The outer calyx lobes are ovate and covered with prominent long hairs. The posticous (the side towards the axil) calyx lobe margins are entire and the apex tapering into a sharp spine. The anticous (the side away from the axil) calyx lobe margins are entire, but the apex does not end in a prominent spine. The bracteoles are imbricated (with margins of structures overlapping like shingles on a roof), enclosing the calyx, they are ovate and the margins entire, both the margins and veins are covered with long white hairs. The apex tapered into a sharp spine.

The seed capsules are elliptic, with 4 seeds per capsule. The ripe fruit capsules, turn brown when mature, opening explosively, shooting out the seeds, when moistened. It flowers in summer and autumn, from December to June.