Stegosaurus (STEG-oh-SORE-us) is a genus of stegosaurid armoured dinosaur from the Late Jurassic period (Kimmeridgian to Early Tithonian) in what is now western North America.

Due to its distinctive tail spikes and plates, Stegosaurus is one of the most recognisable dinosaurs, along with Tyrannosaurus, Triceratops, and Diplodocus. At least three species have been identified in the upper Morrison Formation and are known from about 81 remains. They lived some 155 to 145 million years ago, in an environment and time dominated by the giant sauropods Diplodocus, Camarasaurus, and Apatosaurus.

Stegosaurus was a large, heavily-built and herbivorous quadruped, Stegosaurus had a distinctive and unusual posture, with a heavily-arched back, short forelimbs, head held low to the ground and a stiffened spiked tail held high in the air. Its array of plates and spikes have been the subject of much speculation. Stegosaurus' rear legs were longer and straighter than its front legs, which sprawled out to the sides. The forefeet (the feet of the front legs) had five short, wide toes with short, hoof-like tips. The rear feet had three short, wide toes with hooves.

The spikes were most likely used for defence, while the plates have also been proposed as a defensive mechanism, as well as having display and thermoregulatory (heat control) functions. They may have also been used for mating display purposes. Different species of Stegosaurus had different numbers of tail spikes; Stegosaurus ungulatis had 8 spikes and Stegosaurus stenops had 4 spikes. Some early Stegosaurus had shoulder spines.

Stegosaurus was the largest of all the stegosaurians (bigger than genera such as Kentrosaurus and Huayangosaurus) and, although roughly bus-sized, it nonetheless shared many anatomical features (including the tail spines and plates) with the other stegosaurian genera.

Stegosaurus was up to 26-30 feet long (8-9 metres), about 9 feet tall (2.75 metres), and weighed about 6,800 pounds (3100 kg). Its small brain was only the size of a walnut (weighing roughly 2.5 - 2.9 ounces (70 - 80 grams). Its skull was long. pointed, and narrow; it had a toothless beak and small cheek teeth. Its head was carried close to the ground, probably no more than 3 feet (1 metre) high.

This plated dinosaur was an herbivore (it ate only plants). It must have eaten a large amount of low-calorie plant material each day to sustain its bulk, probably using its toothless beak to get food. There is debate on whether or not Stegosaurus could rear up on its rear legs to forage for vegetation. If it couldn't rear up, it was limited mostly to plants no taller than about 3 feet (1 metre) tall. This would have included ferns, smaller club mosses, cycads, horsetails, and bushy conifers.

It isn't known with any certainly, but possible predators of Stegosaurus include all the large meat-eaters from western North America during the late Jurassic Period, about 156-140 million yeas ago (where and when Stegoaurus lived). Some of these meat-eaters included Allosaurus, Ceratosaurus, Torvosaurus, and maybe even packs of smaller meat-eaters like Ornitholestes.

Stegosaurus, like other Stegosaurians, may have been a herding animal, but this is far from certain.