Barleria wilmsiana is an unarmed shrublet, up to 400 mm high. The stems develop out of a woody rootstock and are glabrous (without hairs); older stems are reddish.

The leaves are opposite, ovate (egg-shaped), subsessile (shortly stalked)  and the base is rounded. The leaf margins are entire and the apex (leaf tip) tapers to a point. The leaves are dull green above and light green beneath. Young leaves have a reddish tinge beneath.

The flowers are solitary in the leaf axils or a few are clustered at the tips of the branches. The flowers range in colour from light to dark mauve. The corolla is 2-lipped, the upper lip 4-lobed and lower lip 1-lobed. Each flower is subtended by modified leaves called calyx lobes. The calyx lobes are narrowly elliptic and the margins are entire. Plants flower from late summer to midwinter, from February to June, during which time flowers appear sporadically.

The seed capsule is turgid at the base and ends in a conical beak. The ripe capsules explode to distribute the seeds.