Kadua knudsenii is a diffuse, scandent, or drooping shrub with stems reaching 10–40 decimeters long, characterized by internodes with a sharp longitudinal ridge. The leaves are simple and opposite, widely spaced along the stems. The leaf blades are broadly ovate, elliptic, or rarely broadly ovate–cordate, measuring 6–10 cm long and 2.5–7 cm wide. The apex is acuminate to caudate, while the base is cuneate to rounded, occasionally somewhat cordate. The lower surfaces are sparsely hirtellous, especially along the veins, with chartaceous blades. The margins are entire, and the lateral veins are ascending, slightly raised on the lower surface, with higher-order venation conspicuously reticulate. The petioles are 0.4–0.7 cm long, with bases slightly connate below the stipules. The stipules are deltate, sharply carinate, measuring 3–4 mm long, adnate to the inner side of the petiole base, with an apical mucro 0.6–1.5 mm long. The flowers are arranged in compound, thyrsoid, terminal panicles up to 30 cm long, with lateral branches initially abortive at the first node of the inflorescence or flowering late. The ultimate branches terminate in open cymes. The inflorescences are sparse to moderately hirtellous throughout, with pedicels measuring approximately 1–2 mm long and glabrous to sparsely hirtellous. The hypanthium is urceolate-turbinate, 1–1.2 mm long, and sparsely hirtellous. The flowers are bisexual (perfect) and pistillate, pollinated by insects. The calyx is 4–5 lobed, with oblong lobes approximately 1 mm long and 0.3–0.5 mm wide, with obtuse apices. The corolla is green, with a reddish-purple tube, fleshy, and salverform. The tube measures 7–9 mm long, and the limb is strongly quadrangular in bud, with an apex that is not depressed. It is 4–5-lobed, with lobes measuring 3–5 mm long, valvate in bud, spreading to reflexed, with reflexed margins and well-developed linear appendages up to 1.5 mm long at the apex. A nectary disc is present. The stamens are as many and alternate with the corolla lobes, inserted just below the corolla lobe sinuses. The anthers are sessile or borne on short filaments, dithecal, and open by longitudinal slits. The ovary is inferior or partly so, with 2(-4) locules. The ovules are few to numerous on fleshy placentas near the middle of the septum. The style is as many as the carpels, cylindrical to filiform, terminal, slender, and densely hirsute in the lower one-third. It is 2-lobed in the upper half, with a bilobed or subcapitate stigma that may be included or exserted. The fruit consists of subglobose capsules that are slightly compressed, measuring 2.8–3.3 mm long and 3.5–4 mm in diameter. The capsules dehisce loculicidally across the disk and later separate up to approximately half of the base. They do not or only very tardily separate septicidally. The endocarp is sclerified, and the disk is nearly flat or slightly depressed, with a groove along the septum, and is short–hirtellous. The seeds are dull dark brown, irregularly cuneiform or subpeltate, minutely rugulose–pitted, with well-developed oily endosperm, though sometimes may be scanty or absent. It occurs in mesic forests between 610-1,100 meters.