Plectranthus ernstii is an erect to decumbent, pleasantly aromatic, semi-succulent herbaceous perennial up to 250 mm tall, branching from the base. The stems are greyish and articulated, i.e. they appear to be jointed, with segments and distinct nodes or joints, and are swollen, particularly at the base where they can reach up to 50 mm in diameter, and become brown and potato-like with age. These swollen, jointed stems make this Plectranthus distinct from all others.The leaves are aromatic and have 6-13 mm long petioles. The leaf blade is semi-succulent, ovate to broadly ovate to somewhat triangular, 12-30 x 10-25 mm, with 3-5 pairs of pronounced rounded marginal teeth. The upper-surface is sparsely covered in short fine soft hairs (sparingly pubescent), and the under-surface is covered in minute hairs and pale to reddish brown gland-dots. The tip of the leaf ends in a blunt, rounded point (obtuse) and the base ends abruptly, as if cut (truncate).