Sicyos waimanaloensis is a climbing annual herb with stems extending several meters long. It is typically glabrous. The leaves are simple and alternate, with blades broadly ovate–cordate or reniform, and exhibit a broad basal sinus. They range from 4–14 cm in length and 4–13 cm in width. The leaves are shallow to deeply palmately 5–7–lobed, with lobes that are triangular to obovate. The apex of the central lobe is long-acuminate and often prone to early breakage. The upper surfaces of the leaves may appear glabrous or sparsely scabrid and sometimes white-dotted upon drying; the lower surfaces are either glabrous or scabrid. Margins are dentate or denticulate. The leaves are petiolate, and instead of stipules, there are 2–3–branched tendrils in the stipular position. The flowers are typically unisexual, with the plant either monoecious or dioecious. Staminate (male) flowers are organized in glabrous panicles ranging from 3–15 cm long, with 2–11 cm peduncles. These flowers are sessile or have pedicels up to 3 mm long. Pistillate (female) flowers number 7–15(–20) per head, with peduncles ranging from 1–9 cm, and are subsessile or on pedicels up to 3 mm long. The calyx consists of 5 sepals inserted on a hypanthium. The corolla of staminate flowers is white or pale yellow, 5-lobed, about 2–3 mm in diameter, with petals usually connate at the base and forming a rotate limb. Staminate flowers feature (2)3(–5) stamens inserted at the base of the hypanthium, with filaments connate into a column and anthers that are curved, flexuous, coherent, and twisted in a head, opening by longitudinal slits. Pistillate flowers lack staminodes. The fruit is green, ovoid-lanceolate, measuring 5–7 mm long and 2–3 mm in diameter, and can be glabrous, puberulent, or pilose, sometimes warty toward the base, tapering gradually into a long beak. It is indehiscent. Each fruit contains one ovate seed, lacking endosperm. It is found in open sites or shrubland coastal communities, especially on talus slopes between 50-650 meters.