Sicyos macrophyllus is a perennial vine notable for its extensive growth, with stems reaching up to 15 meters long and 4 centimeters in diameter. The annual stems are sparsely pubescent, becoming smoother (glabrate) over time, and are characterized by black spots. The leaves are simple and alternate, with broad ovate-cordate blades 7–25 cm long and 6–26 cm wide, featuring a narrow basal sinus. These leaves can be shallow to deeply 3–5-lobed, with broad ovate-triangular lobes. The central lobe has an acute or short-acuminate apex, while the lateral lobes are obtuse. The upper surfaces of the leaves are glabrous, and the lower surfaces are densely hispid or puberulent, with denticulate margins. The petioles measure 2.5–7 cm in length, and the plant has 2-branched tendrils in the stipular position. The flowers are typically unisexual, with the plant being monoecious or dioecious. Staminate flowers form in 8–25 cm long panicles with glandular puberulent pedicels. Pistillate flowers appear in groups of 2–6 per head on peduncles 2–8 cm long. The calyx consists of 5 sepals, and the corolla of staminate flowers is greenish-yellow and 5-lobed, whereas pistillate flowers have a smaller 5-lobed perianth. The fruit is green, ovoid, 13–18 mm long, and 4–6 mm in diameter, with 5–6 ribs, often beaked and indehiscent. Each fruit contains one ovate seed, lacking endosperm. It occurs in wet and subalpine forests on windward slopes between 1,200–2,000 meters.