Sicyos herbstii is a climbing annual herb with glabrate stems that can grow up to several meters long. The leaves are simple and alternate. The blades are broadly ovate with a basal sinus, measuring 3.5–7 cm in length and 5–10 cm in width, and are shallowly palmately 5–7-lobed. The lobes are acute or acuminate, with the central lobe being long-acuminate and the narrow apex often breaking. The upper surfaces are hispid with simple white hairs, while the lower surfaces are glandular pubescent on the central veins. The leaf margins are lobed with denticulate lobes. The petioles are 2–4 cm long and glandular pubescent. Stipules are absent, and tendrils in the stipular position are 2–4-branched. The flowers are usually unisexual (with the plants being monoecious or dioecious) and actinomorphic (radially symmetric). Staminate flowers are in panicles 5–12 cm long, with peduncles 4–9 cm long, glandular pubescent, and pedicels 3–6 mm long, short-hispid. Pistillate flowers are 6–9 per head, with peduncles 0.5–1 cm long, glandular-pubescent, and subsessile or on pedicels up to 1 mm long. The calyx consists of 5 sepals inserted into the hypanthium. The corolla of staminate flowers is greenish yellow, 5-lobed, and 4–7 mm in diameter, with petals inserted on the hypanthium alternate with the calyx lobes, usually connate at the base, and the limb often rotate. The corolla of pistillate flowers is 5-lobed, with the perianth usually smaller than in staminate flowers. The stamens in staminate flowers number (2)3(–5), inserted at the base of the hypanthium, with filaments connate into a column and anthers curved or flexuous, coherent, and twisted in a head, opening by longitudinal slits. Pistillate flowers have staminodes absent. The ovary is inferior, of a single carpel or 3-carpellate with carpels connate into a single 3-lobed cell, with one ovule and parietal placentation. The stigma is capitate and shortly 3-lobed. The fruit is pale brown, ovoid, 5–7 mm long, and 3–4 mm in diameter. It is 5-ridged, with the ridges bearing a few spreading outgrowths 1–2 mm long, pilose with multicellular hairs, and reticulate-hispid between ridges. The fruit is indehiscent and contains one ovate seed. The seeds lack an endosperm. It is found in shrub communities on clay loam between 0–500 m.