Opuntia monacantha is a succulent, thorny shrub that grows almost tree-like with several branches and profusely expanded crown in branches that reaches a height of up to 5 meters. The oval to elongated shoots narrowed at the base are shiny green. They are quite thin and four to ten inches long. The widely spaced areoles have brownish glochids. The straight thorn (rarely two to three are present) is brown and between 3 and 4 centimeters long. The dark yellow flowers reach a diameter of up to 8 centimeters. The pear-shaped, red fruits are thornless and up to 7 centimeters long.