Viola lanaiensis is a small, erect, and typically unbranched or few-branched subshrub, standing 10–40 cm tall. It develops from a taproot and occasionally roots from the lower part of the stem. The leaves are simple and alternate, clustering towards the upper part of the stem. The leaf blades are elliptic-lanceolate, measuring 6–11 cm long and 1.3–2.7 cm wide, with an acuminate apex and an attenuate base. The surfaces of the leaves are glabrous, and the margins are callose-serrulate. The lateral veins of the leaves are arcuate, and the petioles range from 0.5–1.5(–2) cm in length. The stipules are thin and deltate, with fimbriate margins, some of the teeth ranging from 2–6 mm in length. The flowers are axillary, typically occurring as 1(–4) in the upper leaf axils. The peduncles and pedicels are relatively short, and the flowers are bisexual and chasmogamous (plant reproduction in which the flowers open up, enabling cross-pollination). The calyx comprises 5 subequal, oblong-lanceolate sepals, with the lateral ones slightly longer. The corolla is zygomorphic (bilaterally symmetric), composed of 5 distinct, unequal petals, white tinged with purple or marked with purple veins. The stamens are about 3.4–3.7 mm long, with the connective of the lower ones forming a nose-shaped structure. The ovary is superior, 3-carpellate, and 1-celled. It produces capsules as fruit, which are 9–13.5 mm long and demonstrate loculicidal dehiscence. The numerous seeds are brown, obovoid, and have a leathery, dull seed coat with a rounded apex and a fleshy, oily endosperm. It predominantly inhabits ridges and slopes between 850–980 meters.