Phyllostegia haliakalae is a suberect perennial herb with moderately to densely antrorsely strigose and sparsely glandular-dotted stems. The leaves are simple and opposite, with ovate to elliptic-ovate blades measuring 9–22 cm long and 3.7–10 cm wide. The leaf apex is obtuse to rounded or occasionally acute, with a base rounded to truncate or rarely subcordate. The upper surfaces of the leaves are sparsely short-hirtellous to antrorsely strigillose, while the lower surfaces are short-hirtellous to antrorsely strigose and inconspicuously glandular-dotted. The blades are thin, and the margins are crenate. The flowers are (4)6 per verticillaster (inflorescence that resembles a whorl but consists of two crowded cymes on either side of the stem), found in short, compact, compound, terminal, racemose inflorescences approximately 6–8 cm long. The pedicels are 1–3 mm long and densely antrorsely strigose. The calyx is obconical, about 3–3.5 mm long, and enlarges in fruit. The corolla is bilabiate and probably white, with a narrow funnelform tube that is slightly curved, approximately 6–10 mm long, and covered in antrorsely strigose and glandular-dotted hairs. The fruits are nutlets about 2 mm long, 1-seeded, dark green to black, and drupaceous. The seeds have an absent or scanty, oily endosperm. It occurs in mesic gulches between 680–730 meters.