Psychotria mariniana is a small tree species, occasionally reaching up to 20 meters in height, with yellowish to dark brown branches. The leaves are simple and opposite, with narrowly oblanceolate blades to obovate, measuring 5–13 cm long and 1.5–6 cm wide. These leaves feature an acute to obtuse or rounded apex, usually ending in an obtuse, abrupt point, and a base that is typically acuminate or attenuate. The lower surfaces of the leaves are usually glabrous, except for the domatia, which are very conspicuous, especially in specimens from Kaua’i and some from O’ahu, and filled with whitish hairs. The plant has 7–12 pairs of lateral veins and often developed marginal ribs. The petioles range from 0.5 to 3.3 cm in length, and stipules are broadly ovate to obovate, up to 10 mm long. The flowers are borne in inflorescences with a central axis and usually 2–3 nodes of verticillate branching, with robust peduncles 1.2–6.2 cm long. The flowers are insect-pollinated, functionally unisexual, and typically heterostylous, with a 4–6-lobed calyx that is thick, fleshy, and 1.5–4 mm long. The corolla is white, with a 1–2.5 mm long tube and thick, fleshy, reflexed lobes. There are as many stamens as the corolla lobes, with filaments inserted on the corolla tube or throat. The fruit is drupaceous, orange, pyriform to oblong-globose, measuring 10–12 mm in length, with 1-2(-4) pyrenes that are hemispherical in cross-section. Seeds conform to the form of the pyrene, have a well-developed oily endosperm, and a thin seed coat.  It is found in mesic to wet forests between (60–)180-1,220 meters.