Metrosideros polymorpha var. glaberrima is a medium-sized or tall tree reaching heights of 10-15 meters. It is notable for its glabrous morphology, including leaves, stems, inflorescences, and fruits. The bark is usually rough and fissured, peeling off in thick flakes, though sometimes it is smooth and sheds in long, thin strips. Vegetative buds are obovoid, while branches and branchlets are cylindrical. The leaves are simple and opposite, with ovate, elliptic, oblong, or obovate blades, measuring 3.5-7.5 cm in length and 1.5-3.5 cm in width. The apex can be rounded, obtuse, or acute; the base mirrors this variability. Both the upper and lower surfaces of the leaves are glabrous, and the leaves range from chartaceous to coriaceous in texture. Margins are entirely flat. The veins, several pairs of which radiate from the base, are raised, contributing to the distinct intramarginal nerve observed in larger-leaved specimens. Petioles are relatively short, measuring 3-10 mm, and stipules are either vestigial or absent. The flowers appear in inflorescences consisting of 2–5 pairs of cymules, all of which are glabrous. The peduncles measure 7–18 mm in length and 1–3 mm in width, with bracts that are broadly ovate to suborbicular, 5–10 mm long, and 3–5 mm wide. The flowers are actinomorphic (radially symmetric) and can be red, pink, or orange; they are perfect (bisexual). Pedicels are 2–8 mm long and 1–2 mm wide. The hypanthium is 3–7 mm high and 3–8 mm wide. Sepals are rounded to triangular, 1.5–4 mm long, and 1.5–3 mm wide, thin, glabrous, with reddish thinner margins. Petals are oblong, red to salmon-pink, glabrous, and may have slightly ciliate margins. Stamens are 1–3 cm long, and the style extends 1.3–3 cm. The fruit is a glabrous capsule, slightly included to exerted, with a fruiting hypanthium measuring 3–8 mm in length and width. It is typically found in shrublands and mesic to wet forests between 0-2200 meters.