Tyrannosaurus, meaning "tyrant lizard", is a genus of coelurosaurian theropod dinosaur. The species Tyrannosaurus rex (rex meaning "king"), is one of the most well-represented of the large theropods. Tyrannosaurus lived throughout what is now western North America, on what was then an island continent known as Laramidia. Tyrannosaurus had a much wider range than other tyrannosaurids. Fossils are found in a variety of rock formations dating to the Maastrichtian age of the upper Cretaceous Period, 68 to 66 million years ago. It was the last known member of the tyrannosaurids, and among the last non-avian dinosaurs to exist before the Cretaceous–Paleogene extinction event.
A female Tyrannosaurus (called Tyrannosaurus rex [or T. rex for short] in the series) appeared in "Dinosaurs". Martin said that she had really sharp teeth like a monitor lizard because she was a meat eater. Another Tyrannosaurus rex was shown alongside several Triceratops when the Kratt Brothers stated that the canyon they were at used to be home to millions of dinosaurs. In that area, they managed to find a fossil tooth that belonged to a Tyrannosaurus rex.
Another Tyrannosaurus rex appeared in "Giants" along with several Triceratops, a pair of duck-billed dinosaurs, and a male Ankylosaurus (in that order). When Zoboomafoo said that Popper had giant teeth like dinosaurs, Martin agreed, adding that 1 dinosaur who had really giant teeth was T. rex, and that Triceratops had had a giant head plate and horns for protection from T.rex's giant teeth.
A Tyrannosaurus also appeared in "Crocodilian". When the Kratt brothers traveled back 80 million years back to the Cretaceous period a 2nd time to see if there were any giant caimans around at the time, they saw a herd of Triceratops and a T. rex that watched them, who saw the Kratts (who were very close to the window it was standing at). It then went after the brothers. Zoboo tried to do something, but the time machine was malfunctioning. Luckily, Zoboo used his drill to tinker the machine and send Chris and Martin back to the present before the T. rex could eat them.
- Like other dinosaurs in the series, this species is computer animated (due to being extinct), making all individuals of the same species (male and female) look exactly the same.
- Fossils showing this dinosaur were also shown.
- The other episodes showing this species reused the same footage from the first episode to show it.
- The Martinandchrisaurus had the teeth of a Tyrannosaurus.