Allosaurus was a carnivorous "lizard-hipped" dinosaur or theropod. It had a bony red crest above each eye, a strong s-shaped neck. Its teeth were 5-10 cm long and curved backward with serrated front and back edges. Allosaurus' strong back and neck aided its jaws in tearing off chunks of flesh.

It had quite short arms which ended in three long hook-like claws. These were important for attacking and holding on to prey. Allosaurus fed on sauropods and other large game.

Although not really built for speed, it had two sturdy legs and a powerful tail. It was a fearsome predator of its time.

The genus Allosaurus is based on the virtually complete skeleton discovered by M. P. Felch in 1883.

More than 60 complete and partial Allosaurus skeletons have been recovered from the Morrison Formation between Wyoming and New Mexico, in the USA. It is the largest and most common carnivore found in these widespread deposits. At the Cleveland Lloyd Quarry in Utah, USA, Allosaurus is by far the most numerous fossil type and is represented by at least 44 individuals. These ranged in size from adults to juveniles.