Lobelia koolauensis, a member of the Campanulaceae (bellflower) family, is an unbranched, woody shrub 0.3 to 1 m (1 to 3.5 ft) tall. The leaves are inversely lance-shaped to rectangular, 8 to 19 cm (3 to 7.5 in) long, and 1.3 to 2.8 cm (0.5 to 1.1 in) wide. The leaf edges are thickened or curled under, fringed with hairs toward the base, and sharp-pointed at the tip. The flowering stalk is two to six-branched and 40 to 70 cm (16 to 28 in) long. The hairless bracts are lance-shaped to egg-shaped and 18 to 32 mm (0.7 to 1.3 in) long. The calyx lobes are triangular, lance-shaped or egg-shaped, and 10 to 15 mm (0.4 to 0.6 in) long. The corolla is greenish or yellowish white and 50 to 75 mm (2 to 3 in) long. The tubular portion of the flower is curved, with spreading lobes. The fruit is an egg-shaped capsule.