Lobelia grayana is a shrub that reaches a height of 1–2 meters with woody, erect stems, topped by a dense apical rosette of leaves. The leaves are simple, alternate, and linear, measuring 10–24 cm long and 0.5–1.5 cm wide. They have an acute apex and are glabrous on the upper surfaces, while the lower surfaces are densely white tomentose except on the midrib. The margins are entire to minutely callose-denticulate and are revolute. The leaves are sessile or subsessile and lack stipules. The flowers are arranged in unbranched inflorescences that are 10–40 cm long, with 8–12 mm long pedicels and tomentose. Each flower is accompanied by subulate bracts 35–45 mm long. The hypanthium is obconical, 4–6 mm long, and covered in white tomentose. The flowers are perfect (bisexual), with a 5-lobed calyx where the lobes are subulate, 6–10 mm long, and the sepals are fused, forming a tube adnate to the ovary. The corolla is zygomorphic (bilaterally symmetric), dorsally cleft almost to the base, and blue, measuring 28-32 mm long and 3-5 mm wide. It can be glabrous or pubescent, with spirally revolute lobes and a suberect tube. It has five stamens that alternate with the corolla lobes. The staminal column is pubescent, and the anthers are glabrous or pubescent with apical tufts of white hairs. The anthers are dithecal, opening by longitudinal slits, and are coherent but separate after anthesis or remain connate, forming a tube into which pollen is shed. The ovary is inferior and 2-celled, often topped with an annular nectary disk, and the stigmas are 2-lobed, becoming receptive after pushing out pollen through the anther tube. The fruit is an obovoid capsule, 9–10 mm long, and covered in white tomentose. It dehisces at the apex through two valves. The seeds are numerous, smooth, brownish, around 0.5 mm long, and have a straight, short to spatulate dicotyledonous embryo embedded in oily endosperm. It grows on cliffs in subalpine shrubland on Haleakalā, East Maui, at elevations ranging from 1,700-2,300 meters.