Dianella sandwicensis is a coarse, glabrous perennial herb that grows 50–200 cm tall. It has spreading rhizomes close to the ground, approximately 1.5 cm in diameter, forming clumps of stems. The leaves are crowded, 2-ranked, and somewhat rigid, with linear-lanceolate blades 30–100 cm long and 2–3(–4) cm wide, featuring a prominent and keeled midrib. The leaves are rigid and flexible, folded toward the base, with entire or sometimes minutely toothed margins. The plant is sessile and lacks stipules. The flowers appear in panicles measuring (10–)20–40(–60) cm long, open or relatively narrow, with corymbiform branches 5–15 cm long. The cymules are 4–12(–25)–flowered, with secund flowers on recurved pedicels (3–)5–10 mm long. The flowers are bisexual and nodding, with pale blue to white, oblong-ovate, reflexed tepals; the outer ones are 5–6 mm long, and the inner ones are 4–5.5 mm long, with obscurely 5–5-nerved patterns. The plant has six stamens with orange, turgid filament swellings and yellow anthers. The ovary is superior, 3-grooved, 3-celled with 2 to numerous ovules per cell and a filiform style. The fruit is a dark translucent blue to sky blue berry, obovoid to depressed-globose, often somewhat irregular, and about 7–10 mm long. It contains 2–3(4) seeds per cell, which are black, glossy, and variable in shape. The black, shiny seeds range from hemispherical with an angular inner face and a single crest to irregular in outline, angular, and ribbed, with one or both ends tapered, measuring 2–3 mm long, and have a crustaceous seed coat. It is found in open to somewhat shaded sites, primarily in mesic forests, but also in dry shrubland and grassland, on lava, and in wet forests, with an elevation range of 120–2140 meters.