Cyanea koolauensis is a terrestrial shrub growing 1–1.5 meters tall with woody, unbranched, unarmed stems that are either muricate or aculeate. The leaves are simple and alternate, with linear to narrowly elliptic blades measuring 16–36 cm long and 1.5–4 cm wide. The leaf apex is acute to acuminate, and surfaces are glabrous and whitish on the lower side, with margins ranging from callose-crenulate to subentire. Petioles extend 1.5–4.5 cm in length, and the plant lacks stipules. The inflorescences comprise 3–6 flowers, with peduncles 15–40 mm long and pedicels 10–22(–30) long. The hypanthium is obconical to turbinate, measuring 6–12 mm long. The perfect, bisexual flowers feature a 5-lobed calyx, with lobes connate into a truncate or irregularly toothed or lobed sheath, and a zygomorphic (bilaterally symmetric), dark magenta corolla 5–9 cm long. The corolla tube is suberect to strongly arcuate, dorsally cleft about halfway, with connate lobes that spread, are rarely erect, or all downcurved in the same plane. There are five stamens, alternate with corolla lobes, with a glabrous staminal column to sparsely pubescent. Anthers are glabrous or sparsely pubescent, with the lower two featuring apical tufts of white hairs. The ovary is inferior and 2-celled, with axile ovule placentation and a 2-lobed stigma that is nonreceptive until after the style grows through the anther tube and sheds the pollen. The fruit is a globose berry containing numerous tiny seeds, pale brown to black, smooth, shiny, and encasing a straight, short to spatulate dicotyledonous embryo in oily endosperm. It inhabits exposed, cloud-swept wet forests along crests and less frequently in mesic valleys, at elevations ranging from 450-670 meters.