Brighamia rockii is an unbranched caudiciform succulent characterized by its simple, alternate leaves arranged in a dense apical rosette. The leaf blades are 6–22 cm long and 5–15 cm wide, with an obtuse to rounded apex and a cuneate base. The leaf surfaces are glabrous and leathery, with entire to weakly and distantly toothed margins in the upper half. The leaves are petiolate, and the plant lacks stipules. The flowers are presented in suberect or spreading axillary, bracteolate racemes. The peduncles, including the rachis, are 3.5–7.5 cm long, and the pedicels are 0.6–1.2 cm long. These flowers are bisexual (perfect) or occasionally unisexual, resupinate, protandrous, and fragrant, usually comprising 3–8 flowers per raceme. The hypanthium is 9–12 mm long and 5–8 mm wide. The calyx is synsepalous (sepals fused) with a 10-ribbed tube adnate to the ovary, forming a hypanthium, and has five free, valvate lobes shorter than the tube. The calyx lobes are oblong to elliptic, measuring 2.5–8 mm long. The corolla is salverform (narrow tube with petals bent abruptly outward at right angles) and white, with a tube 8–13 cm long and lobes that are elliptic, finely erose, 1.7–3.7 cm long, and 0.8–1.3 cm wide. The plant has five syngenesious stamens (anthers are fused while their filaments are free) included within the corolla tube, with glabrous, dithecal anthers opening by introrse (inward) longitudinal slits, each topped with tufts of white hairs. The pollen is tricolporate, prolate, and ellipsoidal. The ovary is inferior, 2-locular, with axile ovule placentation, a slender, exserted style, and a 2-lobed stigma that becomes receptive after dispersing the pollen. The fruit is a capsule, 13–20 mm long, 7–10 mm wide, and 3–4 mm thick, ellipsoid and compressed, initially fleshy but eventually drying. Each locule of the capsule dehisces through two lateral longitudinal slits. The seeds are numerous, small, pale, ovoid to ellipsoid, and measure 1.1–1.2 mm in length, with a smooth surface and a short to spatulate dicotyledonous embryo embedded in oily endosperm. It occurs on basaltic cliffs near the ocean and at elevations up to 470 meters.