Dietes flavida is a clump-forming perennial with a rhizomatous rootstock. It grows 0.5–0.7 m tall, and can reach up to 1.2 m. Leaves are narrow, lanceolate, 300–500 × 15–22 mm, glaucous.

Stems branching irregularly, bearing long, laminate leaves below and reduced, sheathing, bract-like leaves above. Stem bracts are 30–50 mm long. The inflorescence is a rhipidium, meaning that each successive flower pedicel is produced in a zig-zag pattern, in the same plane, up the stem. The flowers have spathes 40–50 mm long, the outer spathe is about half as long as the inner, obtuse to emarginate. In spring and at regular intervals throughout the summer, it produces pale yellow flowers with brownish markings. A flower lasts for less than eight hours. The outer tepals have brown nectar guides at the base of the limbs and are spotted on the claws. The outer tepals are 30–40 mm long; the claws are 15 mm long; limb horizontal, 15–17 mm wide. The inner tepals are smaller, up to 38 mm long. The filaments of the stamens are free and broadened at the base, 4–6 mm long, and the anthers are 5–6 mm long. The ovary is 10–14 mm long; the style 2–3 mm long with branches that are 8 mm long and 3–4 mm wide and crests that are 5–10 mm long and acute.

The seed capsules are drooping, narrowly ovoid, 30–35 mm long, smooth. It dehisces irregularly when mature, meaning that it does not split along the partitions.

It may be confused with Dietes iridoides, also commonly called wood iris or wild iris, but can be distinguished by its cream-coloured to yellow flowers and its drooping, smooth-walled seed capsules.