Sicyos pachycarpus is a climbing vine, potentially with a perennial rootstock. The stems are slender, up to several meters long, about 1 mm in diameter, and glabrous. The leaves are simple and alternate, with broadly ovate-cordate or reniform blades and broad basal sinus. The blades measure 7–26 cm long and 7–28 cm wide, usually shallowly 5–7-lobed or angled, and rarely deeply 5–7-lobed. The lobes are typically broadly triangular but can be narrowly obovate and lobulate. The leaf apex is usually obtuse and mucronate, with the central lobe abruptly acuminate and the slender apex often breaking early. The leaf surfaces are glabrous or sparsely scabrid on the upper side, while the lower surfaces are scabrid with short white hairs. The margins are denticulate, and the petioles are 1.5–3 cm long. Stipules are absent, and tendrils in the stipular position are 2-branched. The flowers are unisexual, with the plants being monoecious or dioecious and actinomorphic (radially symmetric). Staminate flowers are in pubescent or puberulent panicles 3–12 cm long, with peduncles 2–9 cm long and pedicels 2–3 mm long. Pistillate flowers are in heads of 5–50, sessile on peduncles 0.4–2 (up to 4) cm long. The calyx has five sepals inserted into the hypanthium. The corolla of staminate flowers is greenish yellow, 5-lobed, 3–5 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 3–5-lobed, with the perianth usually smaller than in staminate flowers. Staminate flowers have 2–3 (occasionally up to 5) stamens inserted at the base of the hypanthium, with filaments connate into a column. The anthers are curved or flexuous, coherent, rarely spreading, and open by longitudinal slits. Pistillate flowers lack staminodes. The ovary is inferior, consisting of a single carpel or 3-carpellate with carpels connate into a single 3-lobed cell. There is one ovule with parietal placentation, and the stigma is shortly 3-lobed with recurved lobes. The fruit is brown or black, ovoid, and attenuate, measuring 3–8 mm long (including the beak), compressed or angled toward the base, glabrous or puberulent, and abruptly contracted into a short beak. The fruit is indehiscent, containing one ovate seed, which lacks endosperm. It is found in herb or shrubland coastal communities and dry forests, on coral sand or clay loam, primarily on lower leeward slopes between 0-900 meters.