Plantago hawaiensis is a stout perennial herb known for its robust growth from a short, stout, and woolly stem that can extend several centimeters, covered sparsely to moderately with long reddish-brown hairs. The leaves are basal and arranged in a spiral, featuring narrowly elliptic to oblong-oblanceolate blades that range from 7.5–22 cm in length and (0.7–)1.5–3.2 cm in width. These blades are undifferentiated from the petioles, with an acute or bluntly acute apex and an attenuate base. The leaf surfaces are either glabrous or variably pilose, especially along and near the margins, and are thick and coriaceous. The flowers are on scapes ranging from 20–90 cm long, with spikes measuring (7–)15–28 cm long, either glabrous or moderately pilose. These flowers are densely crowded, particularly towards the base, with each one strongly ascending. The bracts are ovate-elliptic, measuring 2.1–2.6 mm in length, and have a ciliate and sometimes sparsely pilose outer surface. The calyx comprises four subequal, distinct sepals that are elliptic to elliptic-ovate, 1.6–2.2 mm long, and primarily scarious except for the green midrib. The corolla is salverform with four lobes, persistent in fruit, enveloping the upper part of the capsule with spreading lobes about 1–1.1 mm long. It has four stamens, alternated with corolla tubes, and long-exerted anthers. The ovary is superior with two carpels and several ovules, leading to an oblong-ellipsoid capsule fruit, 2.6–4 mm long, which dehisces about two-thirds to the base and is circumscissile at or below the middle. The seeds are 4–6 per capsule, dull black, approximately 1–1.1 mm long, and winged at one end, with a well-developed, firm, and translucent endosperm. It occurs in mesic to dry shrubland environments, often found in cracks within lava between 1,800-1,950 meters.