Opuntia megacantha is a shrub that grows in the form of a tree and reaches a height of 4 to 5 meters. Sometimes they form colonies. The sections are ovate to oblong gray-green to light green 40 to 60 (or more) cm long. The small leaves are green or purple. The small areolas are 4 to 5 cm long and bear small yellow glochids . It has 1 to 5 whitish spines, divergent 2 to 3 cm long. The yellow to orange flowers, reaching a length of up to 8 centimeters. The fruits are 7-8 cm long.