Gynochthodes trimera is a tree that grows to heights of 4–16 meters. Its branches are pale brown, cylindrical, or sometimes quadrangular, and feature numerous warts and lenticels. The leaves are simple and opposite, with oblanceolate to elliptic-oblanceolate blades measuring 12–26 cm long and 3.5–9 cm wide. The upper surfaces of the leaves are glossy and either glabrous or very sparsely puberulent. In contrast, the lower surfaces are dull, usually glabrate, but occasionally puberulent, particularly along the veins and in the axils of secondary and sometimes tertiary veins. Notably, there are conspicuous pubescent tufts in the axils of lateral veins and the midrib. The leaves have entire margins and are supported by 0.7–4 cm long petioles. The stipules are connate and often bear mucilage-producing colleters (multicellular secretory hairs) on the inner surface, which help protect the growing shoot tip. Flowering occurs in terminal or axillary inflorescences with 5-12 flowers grouped in globose heads, and these are sometimes clustered basally when fused in heads or, presented in multiflorous compound dichasia, or rarely single. The flowers are sessile or sometimes pedicellate with peduncles measuring 6–60 mm in length and bracts 3–10(–30) mm long. The flowers are insect-pollinated, bisexual, often heterostylous, with a 2–3-toothed, persistent calyx. The corolla is green with 3–4 lobes, the tube measuring 4–8 mm long, and lobes 4–7.5 mm long, with a nectary disk usually present. The stamens are (2)3–4, included and alternate with corolla lobes; the anthers are partly exserted. The ovary is inferior, typically 2-carpellate but can be up to 5, bilocular with a false dissepiment separating the ovules; the ovaries of all flowers in a head can be basally to completely fused. The style is singular or as many as the carpels, approximately 5-8 mm long, cylindrical, with bilobate, and exserted stigmas that are dry or occasionally wet. The fruit are syncarps, dark green, very firm at maturity, becoming somewhat woody, and measure 2–3.5 cm in diameter with a persistent, conspicuous calyx. The seeds are ellipsoid with well-developed oily endosperm, though endosperm may occasionally be scanty or absent. It is usually scattered, sometimes a dominant element of the vegetation, particularly in certain areas of the Wai’anae Mountains on O’ahu, and found in gulches, on slopes, and occasionally ridges in mesic forests between 350-880 meters.