Coprosma montana is a dioecious or rarely monoecious, many-branched small tree or shrub 2-8 m tall. Its branches are ascending, stiff, and somewhat tortuous, with the youngest displaying puberulence. The leaves are simple and often clustered toward the branch tips. Leaf blades vary in shape from oblanceolate and elliptic–oblanceolate to broadly elliptic–obovate or occasionally suborbicular, ranging in size from 10 to 50 mm in length and 5 to 20 mm in width. The leaf apex ranges from acute to rounded, with an attenuate base. While the upper surfaces of the leaves are glabrous, except for some coarse puberulence near the base, the lower surfaces are entirely glabrous. The leaves are either thin or thick and coriaceous, featuring entire margins and pinnate venation. The slightly winged petioles are up to 12 mm long and puberulent on the upper surface. Stipules are broadly deltate, 2–5 mm long, and connate about halfway up, featuring short-ciliate margins and a puberulent base. The flowers are unisexual, dioecious or rarely monoecious, often heterostylous, and insect-pollinated, typically occurring in groups of 1-2 and 5-6-merous, and are subsessile. The calyx in both male and female flowers is short-tubular and irregularly toothed, with whitish ciliate teeth. The corolla of male flowers is narrowly campanulate, while in female flowers, it is narrowly funnelform; both feature a usually present nectary disk. The stamens are long-exerted with pendulous, dithecal anthers. The fruit is a juicy drupe, orange to reddish-orange, ellipsoid to subglobose, measuring 6–10 mm in length, with a well-developed oily endosperm, though occasionally the endosperm may be scanty or absent. The apex of the fruit is marked by conspicuous persistent calyx teeth, enclosing 2(-4) one-seeded, planoconvex pyrenes. This species predominantly occupies subalpine shrubland, often forming a dominant vegetation component, and can also be found in subalpine woodland and occasionally in mesic forests. It thrives at elevations ranging from 1,100 to 3,050 meters.