Phyllostegia kaalaensis is a robust, erect to decumbent perennial herb with stems ranging from 0.9–5 meters in length, and these stems are glabrous. The leaves are simple and opposite, and they have blades that are ovate and oblong-ovate to oblong-lanceolate, measuring 8–24 cm long and 2.5–10.5 cm wide. The leaf apex is long-acuminate, and the base is broadly cuneate or rarely subcordate. The surfaces are glabrous, often red-tinged or with red veins; the upper surfaces are somewhat glossy, while the lower surfaces are pale, thin, and membranous. The margins are dentate, and the petioles are 2.5–7.5 cm long and glabrous. Stipules are absent. The flowers are arranged in verticillasters, with 6–10 flowers per cluster, in unbranched racemose inflorescences that usually develop terminally on the branch. However, occasionally, an additional pair of lateral inflorescences may appear immediately below. The pedicels are slender, and the peduncles are stouter than the pedicels. The bracts are narrowly ovate and sparsely ciliate, with the lowermost pair slightly larger. The calyx is obconical with inconspicuous veins, glabrous, and has narrowly deltate teeth that are sparsely ciliate with an attenuate apex. The corolla is white, sometimes tinged reddish purple, and variable in size. There are four stamens, with both pairs of similar length or the upper pair slightly longer, ascending the upper corolla lip, included and aligned with the sinuses. The filaments are inserted near the throat and are usually pubescent, while the anthers are dithecal, opening by longitudinal slits with divaricate anther sacs. The ovary is superior and 2-carpellate with basal-axile placentation and one ovule per ovary lobe. The style is included, arising between the lobes, shortly 2-lobed with clavate lobes, stigmatic only at the apex, and one lobe is often rudimentary; the stigmas are two. The fruits are 4–5 mm long nutlets, 1-seeded, dark green to black, drupaceous, with a fleshy exocarp. They are obovoid with slightly winged margins, obliquely attached at the connate base, spreading and exserted from the calyx at maturity. The seeds have scanty and oily endosperm or are without endosperm. It is found in mesic summer dry forests between 374-796 meters.