Haageocereus decumbens grows spread out or ascending and often forms richly branched groups with slender shoots that reach1-2 meters and with a diameters of up to. There are about 18 to 20 low ribs. The one or two yellow to browncentral spines are long. The 25 to 40 and more needle-like yellowishradial spines have a length of. The white flowers reach a diameter of and a length of up to. Fruits are ovoid.