Psychotria greenwelliae is a slender tree species that grows up to 5 meters tall. The stems have branches that are chestnut to grayish brown. The leaves are simple and opposite, with obovate, oblanceolate-oblong, or elliptic-oblong blades, measuring 2.2–14 cm long and 1.1–7 cm wide. The apex is acute to obtuse or sometimes rounded. The base is attenuate to narrowly or broadly cuneate. The surfaces are glabrous or occasionally hirtellous, particularly in the axils of veins. The blades are thick or thin and coriaceous. Domatia are absent or small, often with whitish or brownish hairs within them. The margins are entire, with lateral veins numbering 6–11 pairs, and tertiary veins often conspicuously reticulate. The petioles are 0.2–2.5 cm long. The stipules reach up to 8 mm long and often have elongated apices. The flowers are arranged in inflorescences with a single main axis and 1–2 nodes of verticillate branching. The branches are usually less than 5 mm long and typically terminate in a 3-flowered cymule, with the central and oldest flower sessile on the branches and the two lateral ones on short pedicels. Small, caducous bracts subtend the branches. The flowers are insect-pollinated, usually functionally unisexual, 4–5-merous, and often heterostylous. The calyx is 4–6-lobed, 1–2 mm long, often flared toward the apex, with a truncate apex or minute rounded lobes. The tube is usually shorter than the hypanthium. The corolla is white, with a tube up to 3 mm long, slightly expanded toward the apex, and a glabrous throat. The lobes are 4–6, ovate or lanceolate, 4–6.5 mm long, and strongly reflexed. 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 up to 2.5 mm long at anthesis and are usually inserted on the corolla tube or throat. The anthers are attached at the base, oblong, 1.5–2 mm long, exserted at anthesis in functionally staminate flowers, dithecal, and open by longitudinal slits. The ovary is inferior, occasionally partly so, or rarely superior, measuring 2–4 mm long and 2-celled, with basal placentation. There is one ovule per cell, which is erect and anatropous. The styles are as many as the carpels, reaching 5–6 mm long, terminal, and slender. The stigmas are 2-lobed or capitate, 1–2 mm long, puberulent or papillose, divergent in functionally pistillate flowers, and appressed in functionally staminate flowers. The stigma is either dry or occasionally wet. The fruit is a drupaceous, orange, globose–pyriform drupe measuring 10–13 mm long, often with a broad disk at the apex. The pyrenes number 1-2(-4) per fruit and are hemispherical in cross-section, with a thick and hard endocarp. Each pyrene contains one seed that conforms to the pyrene’s shape. The seed coat is thin and features a large T-shaped invagination on the ventral side. 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 occasionally wet forests between 610-1,280 meters.