Perrottetia sandwicensis is a polygamodioecious shrub or small, slender tree, 2–6(–8) meters tall. Branches are typically red to green, usually hairless or occasionally slightly hairy with short light brown hairs (0.1–0.4 mm long). Leaves are spirally arranged, shiny, dark green with pink, red, or reddish-orange veins and petioles. They are generally ovate, elliptic, oblong-elliptic, occasionally obovate, or broadly elliptic (on Maui), measuring 7–19 cm long and 2.5–9.5 cm wide. The upper surface is relatively smooth and hairless, while the lower surface is somewhat paler and can have sparse to moderately hairy veins, especially on young leaves. The hairs are yellowish-brown and 0.4–0.8 mm long. The leaves have 8–11 secondary veins on each side and serrated margins with short teeth (0.3–1 mm long). The apex can be extended to short-acuminate or sometimes rounded or obtuse. The base is cuneate to obtuse or rounded. Petioles are 1.5–5 cm long and can be sparsely to moderately hairy or hairless. Stipules are linear-oblong, 1–1.5 mm long, and can be either hairless or slightly puberulent. Both male and female inflorescences are axillary or occasionally below the leaves. They are panicle-like, pyramidal, with two to three (rarely four on Kaua‘i) degrees of branching. The peduncle is (0.5–) 1–4 cm for male inflorescences and 3–5 cm for female inflorescences. The axes can be hairless or sparsely hairy with brown hairs (0.1–0.4 mm long). Bracts are typically hairless, triangular-subulate, 0.4–1.0 mm long on primary branches and 0.5–1.0 mm on secondary branches. Pedicels are articulated, 1–3.5 mm long, and have 1–3 bracts measuring 0.5–0.8 mm long. Flowers are borne on hairless or slightly puberulent pedicels. There are five sepals and petals in male flowers, with the calyx being greenish-orange and sometimes red-margined. Sepals are deltate and 0.5–0.7 mm long, with entire margins. Petals are similar in color and shape to sepals but thinner, measuring 0.6–0.8 mm long and 1–1.2 mm wide, with ciliate margins. Stamens have filaments 1.5–1.9 mm long and anthers that are transversely ellipsoid, 0.5–0.6 mm long and 0.6–0.7 mm wide. There is also a pistillode measuring 0.5–0.7 mm. In female flowers, there are five sepals and petals, both with greenish-orange to red coloration. Sepals are deltate, 0.6–1.5 mm long and 0.5–0.7 mm wide, with ciliate margins. Petals are similar in color and shape but thinner, deltate, and 0.6–1.1 mm long. The pistil is ovoid and 1.0–1.5 mm long, with two papillose stigma lobes. Five staminodes are measuring 0.3–0.4 mm. The fruit is a depressed-globose berry (2–)4–6 mm in diameter, initially reddish-green and later ripening to bright red or white with red tinges, and it is hairless. Seeds are (2–)4 in number, triangular-globose, 1.3–1.5 mm long, with a thin aril. The testa (seed coat) is rugose-reticulate, occasionally with transverse wavy lines. It is found in wet forests between 300-1,250 meters.