The Gila Monster ( , HEE-la), Heloderma suspectum, is a species of venomous lizard native to the southwestern United States and northern Mexico. It is a heavy, slow moving lizard, up to 60 cm (2 feet) long, and is the only venomous lizard native to the United States. Its skin has the appearance of black, pink, orange, and yellow beads, laid down in intricate patterns. These beads are small bony plates that form scales, and are known as osteoderms. It is one of only two species of venomous lizard, the other being its close relative the Mexican beaded lizard.

The name "Gila monster" refers to the Gila River Basin in Arizona. Heloderma comes from the Greek words Helos — the head of a nail or stud — and derma, or skin. Therefore Heloderma means studded skin. Suspectum comes from Cope's notion that the lizard might be venomous due to the grooves in the teeth.

Unlike snakes which use hollow upper teeth (fangs), the Gila monster injects venom into its victim through grooves in the teeth of its lower jaw. The teeth are loosely anchored, which allows them to be broken off and replaced throughout their lives. The Gila monster produces only small quantities of its neurotoxic venom, which is secreted into the lizard's saliva. By chewing its prey, however, it tries to put as much of the venom into the bloodstream of its victim as possible. The Gila monster's bite is normally not fatal to humans (there are no confirmed reports of fatalities), but it can bite quickly and holds on tenaciously.