Phytolacca sandwicensis is an erect perennial herb, reaching 0.8–4 meters in height, with the base sometimes becoming woody. The stems are usually sprawling, angular, and often puberulent, especially along the angles. Its leaves are simple and alternate, with elliptic-ovate, elliptic, or oblong-elliptic blades measuring 8–22 cm long and 1.5–9.5 cm in width. The leaves have acuminate or acute apices, broadly cuneate to cuneate bases, and are either glabrous or puberulent, particularly along the veins on the lower surface. The leaf margins are entire and sometimes undulate, supported by 0.8–4.5 cm long petioles. The inflorescence is a raceme, usually 8–22 cm long, bearing moderately to densely puberulent flowers. The peduncles are 2–4 cm long with pedicels measuring 3–5 mm, a single bract and two bracteoles subtend each flower. The flowers are bisexual with a calyx of five rose, pink, or occasionally white sepals, approximately 3–4 mm long. It lacks a corolla but has 7–9 stamens in a single whorl, with dithecal anthers that open via longitudinal slits. The ovary is superior, 5–7(8)-carpellate, with connate carpels forming a depressed-globose structure and one ovule per carpel. The fruit is a dark purple berry, longitudinally ribbed with 5–7(8) ribs, about 4–6 mm in diameter. The seeds are 5-16 per fruit, glossy black, subreniform, somewhat wrinkled, and approximately 3 mm long. The embryo is curved around the perisperm and lacks a true endosperm. It grows in open areas or streambeds within mesic to wet forests between 680-1,980 meters, though sometimes as low as 120 meters.