Rex, often known as Rex the Dinosaur, is a large, green, plastic Tyrannosaurus rex who suffers from anxiety, an inferiority complex ("I just don't think I could take that kind of rejection!"), and the concern that he is not scary enough. Rex's worst fear (after Sid) is that Andy will gain another, scarier dinosaur, but feels better after Buzz gives him a few pointers on how to roar more effectively. He is voiced by Wallace Shawn and by Earl Boen in the Toy Story 2 video game.

In Toy Story, he states that he was manufactured by a subsidiary of Mattel (coincidentally, real-life Rex toys used to be made by Hasbro, but as of 2009 are indeed made by Mattel). In the same film, he seems to know that Woody knocked Buzz out the the window by accident, yet he sides against him. Feeling guilty, he is unhappy happy about Woody's disappearance and later vomits when he sees Buzz's broken arm.

The beginning of Toy Story 2, shows him playing the "Buzz Lightyear: Attack on Zurg" video game, which terminates with Buzz being destroyed by Emperor Zurg, much to Rex's frustration. Nevertheless, thinking his video game experiences have prepared him for real combat, Rex accompanies Buzz, Potato Head, Hamm, and Slinky on their mission to rescue Woody after he is stolen by Al McWhiggin of Al's Toy Barn. When the toys enter Al's Toy Barn, Rex excitedly finds a "Buzz Lightyear" video game strategy guide, only to lose it soon after. In spite of that he is able to give Buzz (the toys take a "Bonus Belt" Buzz instead) pointers from information he has acquired from the manual. At one point, he parodies a scene from Jurassic Park by chasing after the car that the toys drive in Al's Toy Barn, in which Mr. Potato Head spots his reflection in a rear view mirror. Rex is considered to be the heaviest of Andy's toys when he loses his grip on the new Buzz's utility line and pushes the toys to the bottom, causing the new Buzz's strength to give out. Later, Buzz #2 and the toys use Rex as a battering ram to break into Al's apartment. When the toys go down the elevator after Al leaves the room with Woody and the Roundup gang, he witnesses the duel between Buzz #2 and an Emperor Zurg action figure. When Zurg is about to finish Buzz #2 off, he turns away, not bearing to look anymore, but his tail knocks Zurg down the elevator shaft, making him feel overjoyed about finally defeating Zurg, but only for real.

In Toy Story 3, he was seen to be especially sad about Andy's lack of attention to the toys, expressing joy when Andy held him for a few moments. He is ultimately responsible for saving the toys' lives, when Buzz uses his tail to tear out of a garbage bag they were about to be crushed in by a garbage truck. When the toys are donated to Sunnyside daycare, Rex's tail is broken off by the abusive toddlers they are played with. Rex comments, "Andy never played with us like that!" Just as in Toy Story 2, he is depicted as the heaviest of Andy's toys, as his weight combined with Hamn's was enough to prevent a reset Buzz from escaping a box they sat upon. At the garbage dump, Rex was the last to escape the shredder, and barely escaped it by grabbing onto a larger piece of metal than the other toys did to survive. He was seen to be the most horrified at their apparently imminent death in an incinerator, and was also the most solemn about their fate. However, Rex is saved by the alien toys commandeering a huge claw. The toys are finally given to a girl named Bonnie, and Rex becomes close friends with Trixe the Triceratops, who he plays computer games with and is seen together with often.