Phyllostegia knudsenii is an erect perennial herb characterized by its short-hispidulous stems. The leaves are simple and opposite, with ovate blades measuring approximately 11.5–18 cm long and 5.1–9 cm wide. The leaf apex is acuminate, and the base ranges from subcordate to truncate. The leaf surfaces are sparsely hirtellous, and the blades are described as flaccid. The margins are crenate, and the petioles, measuring 4–6.5 cm long, are also short-hispidulous. It does not have stipules. It features flowers arranged 2 or 4 per verticillaster (inflorescence that resembles a whorl but consists of two crowded cymes on either side of the stem) on short, leafless, lateral branches that arise in the axils below the lowermost leaves. These branches form racemose inflorescences 4–6.5 cm long and short-hispidulous. Pedicels are approximately 2–4 mm long, and each flower is subtended by spatulate bracts that are 2–3 mm long. The calyx is broadly obconical, about 2.5 mm long at anthesis, enlarging in fruit, and prominently 10-nerved and hispidulous, typically with five broadly deltate teeth. The corolla is 6–8 mm long, and the tube is narrowly funnelform and usually curved. The plant’s upper lip is either entire or 2-lobed, and the larger lower lip has three lobes with slightly undulated margins. The stamens are four in number, 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 usually pubescent, and the anthers are dithecal, opening by longitudinal slits. The ovary is superior with two carpels and basal-axile placentation, containing one ovule per ovary lobe. The style is singular and included, arising between the lobes and shortly 2-lobed; stigmatic only at the apex. The fruits are nutlets that are 1.5–2 mm long. The seeds contain scanty or absent endosperm, which is oily in nature. It was found in lowland mesic forests at 865 meters.