Sideroxylon polynesicum is a shrub or tree reaching heights of up to 10 meters and diameters of up to 25 cm, featuring rough gray or brown bark and secreting white latex. The stems display young shoots that are grayish brown, exhibiting a cracked and sometimes scaly appearance. Initially, these shoots are densely brown pubescent or tomentose but soon become glabrous. They are marked by numerous prominent, persistent bases of inflorescences with no lenticels present. The leaves are simple and alternate, with blades that vary in shape from elliptic and broadly oblong to ovate or suborbicular, measuring between (2–)3–8(–12) cm in length and (1–)2–4(–6) cm in width. The leaf apex may be rounded, truncate, or obtuse, occasionally slightly emarginate, while the base ranges from acute and cuneate to rounded or truncate. Mature leaves are glabrous on the upper surfaces and feature appressed brown to ferruginous tomentose on the lower surfaces. These leaves are coriaceous with entire margins and brochidodromous venation (venation in which the secondary veins do not terminate at the margin but join to form a series of prominent arches), including a marginal vein. The petioles are (1–)1.5–3(–4) cm long, not or only slightly channeled, and pubescent. Stipules are absent. The flowers occur in small clusters of 1–5 within the leaf axils and on stems below the leaves. The floral base often remains, forming a stout, scaly, woody projection 1–5 mm long. These flowers are scented and can be unisexual (dioecious) or possibly bisexual. They rest on pedicels (2–)4–6(–8) mm long, which are appressed puberulent. The calyx comprises 4–5(6) sepals, broadly elliptic or ovate, 3–5 mm long, appressed puberulent externally and glabrous inside, with an obtuse or rounded apex. The corolla is pale green or whitish, with a rotate shape in pistillate and possibly perfect flowers, featuring lobes (5–)7–9(–12) that are ovate, elliptic, or lanceolate. The fruits are reddish to purple or black at maturity, smooth, and range from globose to ellipsoid or ovoid, measuring 1–2 cm long, sometimes beaked, and glabrous. Each fruit contains a single, 8–10 mm long seed with a smooth, shiny seed coat. These seeds are not laterally compressed and have an obtuse apex. It is found in dry forests and lava slopes between 130-640 meters.