It has dull green, clover-like leaves and white, pea-like flowers with a streaky, triangular, purple nectar guide.

Each leaf further consists of three distinctly glandular, hairless, dull green, inverted egg-shaped leaflets of 7–11 mm (0.28–0.43 in) long and 4–6 mm (0.16–0.24 in) wide, with a wedge-shaped base, a smooth margin and slightly indented at the tip, but with the main vein extending beyond the leaf blade into a sharply hooked point.

[1] The flowers occur in groups of three in the axil of the highest leaves of short new side shoots and each triplet is subtended by a small oblong bract that is quickly shed.

The calyx has a purplish wash, is adorned with glands, more densely on the teeth and carries few short, black hairs, pressed to its surface.

[1] Otholobium spissum differs from O. candicans, which is an open spreading or upright shrub with willowy seasonal shoots (not a densely branched, compact shrub), that sheds last season's short shoots (not retaining them), has narrow leaves with very fine soft hair or hairless with hardly visible glands (not broader hairless leaves with clearly visible glands), flowers without bracts, a white silky hairy or hairless calyx, lilac petals (not small tufts of hairs subtending the small stumpy flowers, a sparsely black-haired calyx and white petals) and chestnut brown seeds (not khaki with brown blotches).

[4] It occurs in the Central mountain renosterveld on Bokkeveld and Witteberg shales at an elevation of about 600 m, in the Western Cape province of South Africa.