Allosaurus was a formidable predator that walked on two powerful legs, had a strong S-shaped neck, and vertebrae different from those of other dinosaurs (hence its name, the ''different lizard''). It had a massive tail, a bulky body, and heavy bones. Its arms were short and had three-fingered hands with sharp claws as long as 6 inches.

Allosaurus was up to 38 feet long and 16.5 feet tall. It had a 3-foot long skull with two short brow-horns and bony knobs and ridges above its eyes and on the top of the head. It had large, powerful jaws with long, sharp serrated teeth that were 2 to 4 inches long.

Gastralia (hanging belly ribs) are thin, fragile ribs that helped support and protect the internal organs (like the lungs) in the middle area of the body. These ribs were not attached to the backbone; they were attached to the skin in the belly area.

The different species of Allosaurus varied in weight. Allosaurus fragilis, A. atrox, and A. ferox weighed about 1.1 to 1.9 tons. A. amplexus was much heavier and weighed about 2.7 to 5.5 tons. Allosaurus was the top predator of the Jurassic.