Plantago pachyphylla is a robust to small perennial herb, typically characterized by stout, unbranched stems that reach a length of approximately 20 cm. These stems are brown and woolly, often covered in persistent old leaf bases, and capable of forming adventitious roots along their length. The simple leaves are either spirally arranged or distinctly 2-ranked and range in shape from oblong-elliptic to elliptic-oblanceolate or lanceolate. Leaf blades measure 1.5–41 cm long and 0.3–8.5 cm wide and are undifferentiated from the petiole. The apex may be acute, bluntly acute, or obtuse, and the base gradually narrows into a winged petiole. The upper surfaces are usually glabrous but can be densely tomentose in some populations (e.g., Wai‘ale‘ale, Kaua‘i), while the lower surfaces range from glabrous to densely covered in pale reddish brown or gray tomentum, often more concentrated along the veins. Leaf margins are entire or minutely denticulate, with (3–)5–11 prominent veins. Petioles may extend up to 13 cm long, and stipules are absent. Inflorescences consist of one to several scapes ranging from 10–80 cm in length, each bearing spikes 5–35 cm long. The spikes vary from glabrous to loosely or densely woolly. The flowers are bisexual (perfect), spaced widely to densely crowded, and ascend strongly to a suberect position. Each is subtended by a lanceolate to ovate bract, 2.5–5 mm long, sometimes pilose or woolly at the base. The calyx has four distinct, subequal sepals that are scarious with a glabrous or sparsely pilose midrib. The corolla is salverform, 4-lobed, and persistent in fruit, with spreading lobes 1–2 mm long. There are four stamens with long-exserted anthers. The superior ovary is 2-carpellate with axile placentation, bearing 4 or 8 ovules. The single terminal style supports protruding stigmas. The fruits are ellipsoid to oblong-ellipsoid capsules measuring (2.5–)2.8–3.5 mm long, dehiscing partway from the apex toward the base. Seeds number between one and six per capsule and are ellipsoid–ovoid, 1.6–2 mm long, and range in color from black and brown to reddish-black or yellowish-brown. It inhabits moss-covered tree trunks, wet forests, bogs, and, on East Maui, alpine grasslands or shrublands, between 700-2,200 meters.