Sicyos lanceoloideus is a perennial vine that can grow up to 15 meters long, with the base becoming more or less woody for several meters and reaching up to 3 cm in diameter. The leaves are simple and alternate, with broadly ovate blades featuring a narrow basal sinus, measuring 4–15 cm long and 7–16 cm wide. The leaves are shallowly and palmately 3–5-lobed, with the lobes being broadly triangular. The apex is acute and mucronate; the surfaces are glabrous with denticulate margins. The leaves are petiolate, and stipules are absent, with tendrils in the stipular position being 2–3-branched. The flowers are typically unisexual, and the plants can be monoecious or dioecious. The staminate flowers are arranged in glabrous panicles 8–20 cm long, with peduncles 5–8 cm long and pedicels 5–10 mm long. The pistillate flowers are grouped 6–10 per head, with peduncles 1.3–3 cm long and either subsessile or on pedicels up to 4 mm long. Both types of flowers are actinomorphic (radially symmetric). The calyx consists of 5 sepals inserted into the hypanthium. The corolla of staminate flowers is 5-lobed, 8–10 mm in diameter, with white, ovate-triangular lobes. The petals are inserted on the hypanthium, alternating with the calyx lobes, and are usually connate at the base, with the limb often rotate. The corolla of pistillate flowers is 3–5(6)-lobed and usually smaller than the staminate flowers. The staminate flowers have coherent anthers, with 2 to 5 stamens inserted at the base of the hypanthium. The filaments are connate into a column, and the anthers are curved or flexuous, coherent, and twisted in a head, opening by longitudinal slits. The pistillate flowers lack staminodes. The ovary is inferior, either of a single carpel or 3-carpellate with carpels connate into a single 3-lobed cell, containing one ovule with parietal placentation and a stigma that is shortly 3-lobed. The fruit is green, fusiform, 13–25 mm long (including the beak), 6–8 mm in diameter, and 3–6-angled. It is beaked and indehiscent, containing one seed per fruit. The seeds are ovate, and the endosperm is absent. It is found on ridges or spurs in mesic forests between 500-1,300 meters.