Remains of a large carnivorous dinosaur were named Tyrannosaurus in 1905, a name that has since become a household word. Based on Greek words, its name means "tyrant lizard king", and refers to its size: at the time, Tyrannosaurus rex was one of the largest known carnivorous land animals. It is still the largest tyrannosaur known.

The holotype specimen was found in 1905, in the Hell Creek badlands of Montana, U.S.A., by Henry Fairfield Osborn, of the American Museum of Natural History.

The most complete skeleton currently known (as of 2008) is at the Field Museum of Natural History in Chicago, and it is popularly named "Sue". Based on this specimen, it is estimated that T. rex reached massive proportions: 13 m (43 ft.) long, 4 m (13 ft.) high at the hips, and a mass of 7 t (7.7 tn.).

These animals were active, aggressive carnivores that ate their prey in large pieces or swallowed them whole, although some scientists think that T. rex was more of a scavenger.

Only about a dozen or so relatively good specimens are known for T. rex, although there is additional fragmentary material that represents about 30 or more individuals. The species has been found in rock strata from Texas to Alberta dating from the last several million years of the Late Cretaceous Period, just before the great extinction event 65 million years ago.

The genus Tyrannosaurus has only the one species, rex, although a very close relative, Tarbosaurus, occurs in Central Asia.

A unique discovery by a local school teacher of relatively complete T. rex, nicknamed "Scotty", occurred in the early 1990s in the Frenchman River Valley of southern Saskatchewan. Associated with this specimen were many plant, invertebrate and smaller vertebrate fossils, along with a rare piece of fossilized dung (coprolite) belonging to the T. rex. Such supplementary evidence can tell scientists much about the environment in which the Tyrannosaurus lived.