Wikstroemia villosa is a tall shrub or tree reaching 3–4.5 meters in height. The young branches are yellow and silky pubescent, becoming glabrate with age. The leaves are simple and opposite, with chartaceous blades that are broadly elliptic to elliptic–ovate, measuring (6.3–)8–13(–15) cm long and (1.8–)3–4(–6) cm wide. The leaf apex is acute to acuminate, and the base is slightly tapering to acute. The upper surfaces of the leaves are dark green, while the lower surfaces are brighter and tomentose to villous, sometimes becoming glabrate. The leaf margins are entire, and the veins are pinnately arranged. The petioles are 6–18 mm long and pubescent, and stipules are absent. The flowers occur on peduncles 2–4 mm long, which are hirsute. The rachis is 5–25 mm long, hirsute, and sometimes furcate, with 1–3 mm long pedicels. The flowers can be bisexual (perfect) or unisexual, yellow, and pubescent, becoming glabrate. The calyx tube of staminate and perfect flowers is 12–15 mm long, with outer lobes 2.5–4.7 mm long and 1.5–2.5 mm wide and inner lobes 2–4 mm long and 1.2–1.5 mm wide. The calyx tube of pistillate flowers is 6–8 mm long, with outer lobes 1.5–2.5 mm long and 1.5–2.2 mm wide, and inner lobes 1.3–2 mm long and 1.2–2 mm wide. There are no petals (corolla). The flowers have eight stamens, with the lower whorl included and the upper whorl sometimes shortly exserted. The ovary is superior, 1-celled, with solitary and pendulous ovules. The fruit is elliptic, measuring 10–13 mm long, about 7 mm in diameter, and drupaceous. The seeds lack endosperm. It is found in wet forests on the windward side of Haleakalā between 1,220-1,950 meters.