Thespesia populnea is a tree or arborescent shrub that grows to a height of 5–10(–20) meters. The bark is gray and shallowly fissured. The leaves are simple and alternate, with ovate to deltate or orbicular blades that are 5–30 cm long. The leaf apex is usually acuminate, and the base is cordate. The surfaces are shiny, yellowish green, and glabrous, usually obscurely glandular punctate, with the lower surfaces having an elongate gland on the midrib near the base. The leaf margins are entire, and the petioles are usually more than half as long as the blades. Stipules are present, caducous, or absent. The flowers are axillary, solitary, or in cymes, often forming spicate to paniculate or even head-like inflorescences, with or without an involucel subtending the calyx. The bracts are distinct or basally connate. The flowers are bisexual (perfect), with a cuplike calyx composed of connate sepals, essentially truncate, 8–12 mm long at anthesis, valvate in bud, and enlarging and flattening in fruit. The corolla is actinomorphic (radially symmetric) to moderately zygomorphic (bilaterally symmetric), with five yellow petals that usually have a maroon spot at the base. The petals are obliquely obovate, distinct from each other but adnate at the base to the staminal column, crinkled apically, and 4–7 cm long. There are 5 to numerous monadelphous stamens, forming a yellow staminal column up to 3 cm long, usually included, and densely antheriferous for most of its length, with a 5-toothed apex. The anthers are monothecal, and the pollen is globose and echinate. The ovary is superior and (3–)5-celled, with carpels borne in a single whorl or rarely seemingly superposed whorls. There are 3 to several ovules per cell with axile placentation. The style exceeds the staminal column, is unbranched, 5-grooved, and twisted, with decurrent stigmas. The fruit is a depressed-globose capsule that is leathery, obscurely 5-angled, and 2.5–4.5 cm in diameter, dehiscing irregularly with age. The seeds are grayish brown, 13–15 mm long, glabrate to densely brown pubescent, angled-turbinate, and with or without endosperm. It occurs in protected coastal sites at elevations between 0-275 meters.