Micranthocereus purpureus typically grows unbranched with columnar shoots, reaching heights of over 3 meters and diameters up to 12 centimeters. It has 12 to 25low, wide ribs and large, closely spaced areoles covered in white wool. The plant features 4 to 10 brown central spines up to 5 centimeters long and 15 to 20 needle-like white radial spines up to 10 millimeters long. The cephalium, up to 1 meter long and 12 centimeters wide, is composed of grayish white wool and reddish brown to black bristles up to 2 centimeters long. The flowers range from pink to white and are 4 to 5 centimeters long. The red fruits, spherical to top-shaped, grow up to 2.5 centimeters long.