Psychotria mauiensis grows as shrubs up to 4 meters tall or as trees reaching 12 meters in height. The stems have branches that are grayish brown, chestnut brown, or yellowish brown. The leaves are simple and opposite, with blades broadly obovate to oblanceolate, elliptic, or suborbicular, measuring 1.8–13 cm long and 1–9.3 cm wide. The apex is obtuse or rounded, while the base can be acute, rounded, or subtruncate. The surfaces are glabrous or pubescent, with blades ranging from membranous to coriaceous. Domatia are absent or inconspicuous. The margins are revolute or sometimes flat, with 4–10 pairs of lateral veins. The leaves may be sessile but are usually supported by 0.5–2 cm long petioles. The stipules are deltate–ovate to broadly obovate, reaching 6(–15) mm long. The flowers are arranged in inflorescences with a single main axis and 1–2 nodes of verticillate branching. The branches often further divide divaricately or verticillately, with peduncles and branches that are either glabrous or pubescent. The ultimate branches terminate in a 3-flowered cymule, with peduncles measuring 1–7.4 cm long. The lateral flowers of each cymule are often on small pedicels, and the branches are subtended by small bracts. The flowers are insect-pollinated, functionally unisexual, usually heterostylous, and 4–5-merous. The calyx is 4–6-lobed, measuring 2–2.8 mm long, and is either glabrous or sometimes pubescent or hirtellous, with a truncate apex or short obtuse lobes. The tube is usually shorter than the hypanthium. The corolla is white and 4–6-lobed, with a 1–2 mm long tube that is somewhat expanded toward the apex. The throat is glabrous, and the lobes are often thick, broadly ovate to lanceolate, 1.2–2.9 mm long, reflexed at anthesis, and either glabrous or occasionally pubescent on the lower surface and at the apex. A nectary disk is present at the apex of the ovary. The stamens are as many as and alternate with the corolla lobes. In staminate flowers, the filaments reach 1 mm long at anthesis and are usually inserted on the corolla tube or throat. The anthers are attached at the base, exserted at anthesis in functionally staminate flowers, ovate–oblong, 0.7–0.9 mm long, dithecal, and open by longitudinal slits. The ovary is inferior, occasionally partly so, or rarely superior, and is 2-celled with basal placentation. Each cell contains one erect, anatropous ovule measuring 1.5–2.5 mm long at anthesis. The style is either single or as many as the carpels, measuring 0.5–2.5 mm long, glabrous or pubescent, terminal, and slender. The stigmas are 2-lobed or capitate, divaricate, 0.6–2 mm long, glabrous or pubescent, and either dry or occasionally wet. The fruit is oblong–pyriform, measuring 9–15 mm long, with the disk protruding somewhat from the short persistent calyx. Each fruit contains 1-2(-4) pyrenes, which are hemispherical in cross-section and have a thick and hard endocarp. Each pyrene contains a single seed that conforms to the shape of the pyrene. The seed coat is thin, with T-shaped invaginations on the ventral surface. The endosperm is otherwise not ruminate but carnose. The embryo is small with flat, thin cotyledons. The seeds have well-developed oily endosperm, though it is occasionally scanty or absent. It is found in mesic to wet forests between 215-1,470 meters.