Sanicula sandwicensis is a perennial herb, stout and erect, reaching 20-70 cm. It grows from a caudex linked to a vertical fusiform root, featuring a caulescent stem that is solitary and profusely branched above. The basal leaves are numerous and orbicular, 3–12 cm wide, with the median segments cuneate-obovate and the lateral ones obliquely rhombic and biparted. These leaves are coriaceous, with palmately 3- or 6-parted margins near the petiole and spinulose-serrate edges. The cauline leaves, reduced in size upwards, are palmately divided and laciniate lobed, transitioning from petiolate to nearly sessile along the stem. Basal petioles are slender, 1 to 33 cm long, and narrowly sheathing at the base. The flowers are arranged in terminal clusters, resembling umbels with 2–5 pedunculate, capitate umbels and additional solitary to clustered umbels in the upper leaf axils, each bearing around 20 flowers. Peduncles are 1.5–4 cm long, and the involucre consists of 8–10 inconspicuous, linear-lanceolate bracts, 2–4 mm long. It has unisexual flowers, 3–7 perfect subsessile flowers, and about 15 staminate flowers with 2 mm long pedicels. The calyx teeth are ovate, about 1 mm long, and acuminate. The corolla has five yellow petals, five stamens attached to an epigynous disk, and a two-celled inferior ovary with one ovule per cell. The styles, swollen at the base into a stylopodium, are twice the length of the calyx lobes and recurved. The fruit is ovoid, 3.5–4 mm long, and about 3 mm wide, with subterete mericarps covered in stout, uncinate, strongly bulbous prickles. The seeds, one per mericarp, are contained within a broadly concave commissural surface with a linear scar, surrounded by small, numerous vittae. It is found in scattered subalpine shrubland and woodland on the slopes of Haleakalā in East Maui and the Mauna Kea, Mauna Loa, and Hualālai regions of Hawai‘i between 2,000-2,600 meters.