Canavalia kauaiensis is a perennial climber with twining branches. The leaves are compound and alternate. Leaflets are broadly elliptic to broadly ovate, sometimes ovate, and measure 6.5–15 cm long and 3.7–7.5 cm wide. The apex is bluntly short-acuminate, and the base is broadly cuneate to rounded. Surfaces are sparsely appressed pubescent to glabrous, and the leaflets are chartaceous (papery). Leaflet margins are entire. Leaves are petiolate, and stipules are present but small and caducous. Leaflet stipels are minute. The flowers occur in axillary pseudoracemes 3–9 cm long, with 8–14 flowers. The flowers are papilionaceous (pea-like), with caducous bracteoles. The calyx is 20–22 mm long, sparsely appressed pubescent or glabrate, with a shallowly concave upper edge. The upper lip of the calyx is 10–13 mm long, and the two lobes are connate except at the distal 2–3 mm. The lowest lobe is subulate, 3–4 mm long. The calyx is bilabiate (two-lipped), with 5 lobes—the upper lip is large and has two connate lobes, while the lower lip is trifid (split into three). The corolla is magenta to dark purple, with the standard petal featuring a white spot and streaks at the base, measuring 35–38 mm long. The wing petals are 35–36 mm long, and the keel petals are 39–40 mm long. There are 10 stamens, with the upper one distinct at the base and loosely adhering to the other nine, which are connate into a tube. The ovary is superior and stipitate, with 5–20 ovules. The style is incurved and glabrous, and the stigma is small and terminal. The fruit is a compressed pod, 13–18 cm long and 2.8–3.5 cm wide. The pods are appressed pubescent when young but soon become glabrate. Each valve has a longitudinal sutural ridge and an additional longitudinal ridge (3–)4–9 mm below the upper suture. The seeds are yellowish to reddish brown, oblong-elliptic, and strongly compressed, measuring 20–24 mm long and 12–15 mm wide. The hilum (seed scar) is 13–16 mm long. Seeds are transversely arranged and ellipsoid, with an elongated oblong hilum and a small, papery, persistent rim-aril. It is scattered in diverse mesic forests and those degraded by invasive species such as guava or Lantana between 180–800 meters.