Kadua st.-johnii is a suffrutescent, decumbent, and sparsely branching plant. It features woody stems that can extend up to 30 cm long, adorned with a corky, longitudinally grooved epidermis and marked by corky rings and the connate bases of petioles. These stems often remain obscured by congested, persistent dead leaves, displaying a subscorpioid branching pattern where new branches grow from axillary buds just below the inflorescence, each terminating in further inflorescences. The stems may reach a 1–2 cm diameter at the base and display a light brown to whitish color. The leaves are simple and opposite, with broadly ovate to broadly elliptic blades measuring 5.5–15 cm in length and 3.5–7.5 cm in width. They are characterized by a minutely white–scabrous surface, fleshy texture, and a dark, shining green color on the upper side that contrasts with a glaucous, vein-highlighted lower side. The leaves are sessile or subsessile, with entire margins and strongly ascending lateral veins. The stipules are broad, approximately 3-3.5 mm long, ending in about 1.5 mm long mucro. The flowers emerge on scapes approximately 7–15 cm long, forming corymbose cymes with 5–18 mm long pedicels. These cymes are subtended by ovate–cordate bracts up to 5 cm long. The flowers are bisexual and perfect, featuring a green, fleshy, urceolate corolla about 5–8 mm long. The calyx is 4-5-lobed with foliaceous, broadly ovate lobes that enlarge significantly as the fruit develops. The stamens, inserted near the constricted neck of the corolla, feature dithecal anthers opening by longitudinal slits. The ovary is inferior or partly so, with a cylindrical style to filiform and pilose toward the base. The fruit is dry, ranging from flattened-lentiform to depressed subglobose, with persisting and accrescent calyx lobes that fenestrate and spread, attaching at the fruit’s equator. The fruit dehisces loculicidally across the disc, with pyrenes that later separate by a septicidal slit. The seeds are dark brown to nearly black, irregularly angled, and either papillose or granulate, containing a well-developed oily endosperm. However, the endosperm is occasionally scanty or absent. It is found in the rocky crevices of basalt sea cliffs between 3 – 45 meters.