Coprosma longifolia is a shrub or tree 2-5 m tall. The young stems of this plant are smooth and glabrous. Its leaves are arranged in a whorled pattern, with three per node. The leaf blades are oblong-lanceolate, lanceolate, or elliptic-lanceolate, with lengths ranging from 65 to 115 cm and widths from 13 to 24 cm. The apex of each leaf is short-acuminate to acute, while the base narrows down to attenuate or narrowly cuneate. These leaves are glabrous, somewhat thick, and chartaceous, featuring smooth margins and pinnate venation. The petioles extend 6 to 20 mm in length. Stipules are membranous, measuring 9 to 12 mm in size, and are connate to about a quarter of their length, quickly shedding as the young shoot grows, with glabrous margins that are occasionally dentate with a few small appendages, especially noted in specimens from Mt. Ka’ala in the Wai’anae Mountains. The flowers are in clusters of (3–)6–12, with a 7–8-numerous configuration, supported by 4–12 mm long, glabrous peduncles. It is dioecious, with distinct male and female flowers often heterostylous and pollinated by insects. The calyx in male flowers is cup-shaped with acuminate teeth, while in female flowers, it is also cup-shaped, sometimes slightly bilabiate. The corolla of male flowers is campanulate, whereas it is narrowly funnelform in female flowers. The plant’s reproductive structures include stamens inserted at the base of the corolla tube, with long-exerted filaments, and an ovary that is typically inferior, containing one ovule per cell. The fruit is a juicy drupe, reddish-orange in color, and ellipsoid, measuring 6–8 mm in length, featuring conspicuous persistent calyx and containing one to two planoconvex pyrenes. The seeds are endowed with a well-developed oily endosperm, although in some instances, the endosperm may be scanty or absent. This species predominantly inhabits mesic to occasionally wet forests, thriving at elevations ranging from 360 to 1,200 meters.