The American iguana is 1.6 meters long and can be more than 2 meters long, with its tail accounting for two-thirds of its total length. The body is flat on the left and right, and there are spiny scales on the midline of the back, which are arranged in an hyena shape. There is a laryngeal pouch with a wrinkled sac in the throat. There is a row of thorns along the back to the tail.
The American iguana has top eyes on its head, like lighter scales. There are small and raised scales behind the neck, and large and round scales on the cheeks. The males are all brown and the females are light green gray. The green body of the American iguana can help them hide from large predators.