Barleria virgula is a slender shrublet up to 300 mm high. The stems grow out of a woody rootstock. Older stems are woody and reddish in colour and covered with simple white hairs. The leaves are opposite, linear, with scattered hairs on the midrib; margins entire; petiole short, with narrowed leaf tips. Flowers solitary in the leaf axils (angle between the stem of the plant and the leaf stem), mauve, with purple markings in the throat. The corolla (flower petals) is 2-lipped, upper lip 4-lobed and lower lip entire. Corolla tube yellow. Each flower is subtended by modified leaves called calyx lobes .The calyx lobes are elliptic; margins spiny, toothed and apex ends in a sharp point. The secondary bracts are called the bracteoles; they are lanceolate, thorny like and situated at the base of each flower. Plants flower in late summer, autumn and winter, from February to June, during which time flowers appear sporadically.

The seed capsules are elliptic, with a short beak. When the ripe capsule becomes moist, they burst open to distribute the seeds. There are 4 seeds per capsule.