The Tyrannosaurus rex, Tyrannosaur, or simply T. rex, is a formidable predator from the Late Cretaceous period. It is by far the most well known dinosaur species to this day, known by its reputation as the king of the dinosaurs. Tyrannosaurus was a theropod belonging to its namesake family Tyrannosauridae, which evolved during the Cretaceous and inhabited the super continent of Laurasia (Which today forms North America, Europe and Asia). They are recognized by their short, two-fingered arms and fearsome jaws.

InGen’s Tyrannosaurus are massive predators with enormous boxy heads lined with banana-sized serrated teeth. A pair of passages called choanae are present on the roof of their formidable mouths. A T. rex’s bulging jaws taper into a relatively thin snout, allowing their piercing yellow eyes to glare directly forwards. Their torsos are robust and barrel-like, while their powerful legs are exceptionally muscular. Though small, their arms are muscular and capable of grasping. The thick skin of a Tyrannosaur is wrinkly with small pebbly scales, especially on the head and neck. Small round scutes line the sides of their necks while crocodilian belly scales are present on the underside of this animal. Keratinous lacrimal “brows” above the orbits give Tyrannosaurus rex a permanently angry stare.

Perhaps the mightiest carnivore to roam the Earth, Tyannosaurus rex is an ambush predator, silently hiding behind the treeline before waiting for the perfect moment to strike. They typically kill by violently shaking their prey within their massive, bone-crushing jaws. Tyrannosaurs are also opportunistic scavengers. While the olfactory senses of T. rex are only rivaled by the Turkey Vulture, capable of picking up scents from 10 miles away, they are believed to have visual acuity based on movement. Some suggest this may be due to genetic hybridization with frogs or environmental factors.

Tyrannosaurs are surprisingly social creatures. They are monogamous animals believed to mate for life. Highly protective, Tyrannosaur parents are known to rear young well into adolescence and even adulthood. They take an active role in teaching offspring predatory behavior, presenting injured animals to their young to practice hunting. Parents will frequently allow their progeny to eat first. T. rex nests typically consist of branches, carcasses, and debris tediously collected by a parent surrounding a shallow depression. These carnivores have also been observed engaging in craniofacial biting behavior, both for dominance and even play. Despite their sheer brawn, Tyrannosaurus is a highly complex and intelligent animal.

InGen cloned at least seven Tyrannosaurs on Isla Sorna sometime in the 80s. Hammond nicknamed them "The Seven Rulers of the Island.” One of the seven individuals, an adult female, would be picked to be transported to Isla Nublar for the 1994 opening of Jurassic Park. While her paddock was designed to house a juvenile, no juvenile was ever introduced. This individual would remain on the island for the next two and a half decades after the fall of John Hammond’s dream in 1993.

With the failure of the park and the eventual bankruptcy of InGen, the company sought to recapture living specimens from Isla Sorna in the attempt to open Jurassic Park: San Diego. During Peter Ludlow's expedition to Isla Sorna in 1997, a Tyrannosaurus family would be encountered, consisting of an adult male and female, as well as their male infant. The two males were captured and transported to the mainland. Due to poor animal welfare decisions, the adult would escape confinement from the transport vessel, wrecking havoc across San Diego, CA. While the male Tyrannosaur and his infant were safely returned to Isla Sorna, the San Diego Incident would shock the world with the revelation that dinosaurs were roaming the Earth once again.

During the construction of Jurassic World in 2002, the lone Tyrannosaurus rex on Isla Nublar was recaptured. This mature female specimen would be exhibited at the "Tyrannosaurus Rex Kingdom" attraction where she would be periodically fed goats. Park staff would eventually nickname her "Rexy" (as seen in Evolution of Claire). Rexy would live her life in captivity at Jurassic World for a decade before being set free to confront the Indominus rex during the 2015 incident.

Two Tyrannosaur individuals consisting of a mature female and her adult offspring were smuggled by Mantah Corp off Isla Sorna prior to mid 2016. They were held in a technologically advanced private facility for paleoneurobiological research conducted by Dr. Mae Turner.

In 2018, Isla Nublar's lone Tyrannosaur would be captured by animal traffickers during Mt. Sibo's eruption. Rexy was shipped to Lockwood Manor in Northern California to be illegally sold in an underground auction. The auction ended disastrously and Rexy was set free to roam the forests of California. She managed to evade the Department of Fish and Wildlife until 2022. Rexy was transported to Biosyn Sanctuary where she was reintroduced to other Tyrannosaur individuals from Isla Sorna.