Unlike many dinosaurs, Euoplocephalus tutus is known from a surfeit of specimens, including 15 complete or partial skulls and one nearly complete skeleton.

Euoplocephalus was a ponderous, clumsy creature that weighed about 2.5 t (2.8 tn.) and was about 5.5 m (18 ft.) long. It was a squat animal whose trunk was nearly immovably bound by a mass of bony armour. Even the eyelids were protected by moveable, bony plates. Its name, which is based on Greek words, means "well armoured head".

Canadian palaeontologist Lawrence Lambe first described this ankylosaurid dinosaur 1902, giving it the name Stereocephalus, but he renamed it in 1910 to its current name. There is one species in the genus.

On the end of its tail, Euoplocephalus had a defensive club that was made of four fused, bony growths. Remains of armoured dinosaurs that had tail clubs were first discovered in North America, although a greater variety is now known from central Asia.

The suppleness of its forelimbs suggests that Euoplocephalus could dig. Complicated cranial sinuses suggest a keen sense of smell. The small brain and weak jaws complement the general impression of a sedentary animal that fed on fleshy vegetation and tubers close to the surface of the ground.

Euoplocephalus tutus lived during the Late Cretaceous Period, between approximately 80 and 70 million years ago. It occurred in some abundance in what is now Alberta and Montana.