Lobelia niihauensis is a shrubby species characterized by woody prostrate stems extending 20–40 cm long. These stems are branched at or below the middle, with each branch terminating in an apical rosette of leaves. The leaves are simple, alternate, and range in shape from narrowly elliptic to oblanceolate, measuring 7–15 cm in length and 0.7–1.8 cm in width. The leaf apex is acute to acuminate, and surfaces can be either glabrous or pubescent. The margins are entire to callose-serrulate and can be flat, undulated, or revolute. The leaves are sessile or subsessile, and stipules are absent. The flowers are in unbranched inflorescences 12–15 cm long, with 8–25 mm pedicels. These inflorescences can be either glabrous or pubescent, and each flower is subtended by filiform, linear, or subulate bracts that are 2–10 mm long. The hypanthium is obconical to hemispherical, measuring 2–5 mm long, and can be either glabrous or pubescent. The flowers are bisexual (perfect) and exhibit a calyx with five linear to triangular lobes measuring 1–5 mm in length, and can be either glabrous or pubescent. The sepals are connate with a tube adnate to the ovary. The corolla is zygomorphic (bilaterally symmetric) and dorsally cleft nearly to the base. It is magenta, measuring 22-36 mm in length and 3-5 mm in width, with a texture that can be either glabrous or pubescent. The lobes of the corolla are spirally revolute, and the tube is suberect. There are five stamens, which alternate with the corolla lobes. The staminal column may be glabrous or sparsely pubescent, and the anthers are glabrous, with the lower two possessing apical tufts of white hairs. These anthers are dithecal and open by longitudinal slits. They are coherent but capable of separating after anthesis or may remain connate, forming a tube into which pollen is shed. The filaments can be distinct or connate above and are attached to the epigynous nectary disk or the base of the corolla. The ovary is inferior, typically 2-celled, and often topped with an annular nectary disk. The ovule placentation is axile, or when the ovary is 1-celled, there are two parietal placentas. The stigmas are 2-lobed, initially appressed, and nonreceptive as the style grows through the anther tube to push out the pollen. Afterward, the stigmas spread apart and become receptive. The fruit is a capsule, ovoid in shape, and 6–8 mm long. It can be glabrous or pubescent and opens at the apex by two valves. The seeds are numerous, smooth, brownish, and about 0.5 mm long, containing a straight, short to spatulate dicotyledonous embryo embedded in oily endosperm. It is found on exposed mesic to dry cliffs between 125–725 meters.