Kadua st.-johnii is a succulent herb in the Rubiaceae (coffee) family with slightly woody, trailing, quadrangular stems up to 30 cm (1 ft) long. The fleshy leaves are clustered toward the base of the stem and are broadly ovate to broadly elliptic, 5.5 to 15 cm (2 to 6 in) long and about 3.5 to 7.5 cm (2 in) wide. Clusters of flowers are borne on 7 to 15 cm (3 to 6 in) long flowering stems. The leafy, broadly ovate calyx lobes are about 3 to 4 mm (0.1 in) long and wide, enlarging in fruit to about 8 to 11 mm (0.4 in) long and wide. The green petals are fused into a tube that is 5 to 8 mm (0.2 in) long and wide. The fruit consists of kidney-shaped capsules with dark brown to blackish angular seeds.