Allosaurus was typically around 30 feet (10 meters) long and 2 tons in weight, although some individuals may have reached 40 feet (13 meters) and 5 tons. Like most meat-eating dinosaurs, Allosaurus had jaws lined with long, sharp teeth that had small serrations, like on a steak knife, running down the front and back. The skull of Allosaurus was somewhat flexible, and some paleontologist think that this might have allowed the dinosaur to swallow very large chunks of meat or to withstand the stress of the victim wriggling in its jaws. The three-fingered hands of Allosaurus were tipped with long, curved claws shaped like the talons of eagles. Backed by strong arm muscles, these claws were well adapted to grasping a struggling plant eater. The claws of the three main toes were not as strongly curved but may have helped hold the prey while the dinosaur fed. Like all advanced meat-eating dinosaurs, Allosaurus had a wishbone.

The basic allosaur body design endured a very long time. The small Cryolophosaurus was present in the Early Jurassic, while Acrocanthosaurus and Neovanator were found in the Early Cretaceous. Two giant Mid-Cretaceous allosaurs, enormous Giganotosaurus and Carcharodontosaurus, the largest predatory dinosaurs known. However, these huge forms died out long before the end of the Mesozoic and were replaced by the somewhat smaller, but more sophisticated tyrannosaurids, such as Daspletosaurus, in North America and Asia, and advanced ceratosaurs in Europe and the southern continents.