Captain Rex (designation number CT-7567) is a fictional character in the Star Wars franchise created by George Lucas. He was introduced as a main character in the animated The Clone Wars 2008 film and the related television series of the same name. Rex is a clone trooper of the Grand Army of the Republic, cloned from the Mandalorian bounty hunter Jango Fett, and serves the Galactic Republic under the command of Jedi General Anakin Skywalker and Jedi Commander Ahsoka Tano. Since his introduction in The Clone Wars, he has also appeared in the 2014 Star Wars Rebels television series, the 2021 Star Wars: The Bad Batch television series, the 2022 Tales of the Jedi television series and various spin-off media. Like all clone troopers in The Clone Wars, Rebels, The Bad Batch and Tales of the Jedi, Rex is voiced by Dee Bradley Baker. In Ahsoka, Rex made his live-action debut in the episode "Part Five: Shadow Warrior", voiced by Temuera Morrison in sequences set during the Clone Wars.

He is physically distinguished from other clone troopers by his close shaved blond hair, and he is identified by blue markings on his armor and by stylized hawk eyes (also known as jaig eyes in-universe) painted on his helmet. Rex was also distinguished by his use of twin DC-17 blaster pistols in combat. He is described as a reliable and exemplary soldier, considered by Anakin to be his "first-in-command". Rex is characterized as believing it is his duty to not only carry out orders but also to protect the men under his command; however, his belief in the Republic wavers and changes over the course of the series.

Initial development for The Clone Wars film chose Alpha-17, featured in Star Wars: Republic, to be the primary clone trooper character. However, Lucas felt this created too much alliteration with the existing principal cast, and a new character was created instead. Rex was developed as the central personality to which the troopers around him aspired and featured in story arcs exploring his character development. Rex became a fan favorite, consistently placing in fan polls and Star Wars character ranking lists.

Initial development for The Clone Wars film chose Alpha-17, featured in the Star Wars: Republic comic, to be the principal clone trooper character. However, because Anakin Skywalker, Ahsoka Tano, and R2-D2 (Artoo) were also principal characters, Star Wars creator George Lucas felt Alpha's name created too much alliteration between the main cast, and a new character, Captain Rex, was created instead. The name "Rex" was chosen by Lucas. Previously, the name Captain Rex was used for the error-prone rookie droid pilot RX-24 of Star Tours, who replaced an earlier concept of an unpredictable and risk-taking Clone Wars veteran.

Voice actor Dee Bradley Baker voices every the clone trooper characters in The Clone Wars, including Rex. To differentiate the characters, Baker considered the personality traits of every clone, choosing one or two adjectives for each, and recorded each character separately. To aid with the vocal performance, Baker and supervising director Dave Filoni developed a "clone triangle" that arranged the characters by personality. Rex, characterized as the central personality type to which the other clones aspired, was placed in the center, and the other clones, who represented an excess of certain traits relative to Rex, were placed around the edges of the triangle.

Filoni wished most for the series to address the fates of Rex and Ahsoka because they do not appear in Star Wars: Episode III – Revenge of the Sith, which was released before The Clone Wars but is set after the series' timeline. In April 2011, he hinted that the two characters' stories were possibly connected and remarked: "A lot of people say they want to know more about [Ahsoka], and along with that goes Rex." He also voiced the production team's conscious effort to write story arcs for Rex: "I think one of the other things is we have such a great development of Rex that we really want to keep telling Rex's story and things centered around Rex."

The official Star Wars Databank describes Rex as a "no-nonsense, by the book" and "exemplary clone trooper, brave under fire and dedicated to the men serving under him" who must adjust to his "maverick" ,"headstrong" and "brazen" Jedi commanding officers. His training is described as having instilled a belief that a soldier's highest calling is duty, though his belief wavers over the course of the Clone Wars and he comes to question the purpose of the conflict. The entry characterized Rex as coming to admire and even imitate Anakin Skywalker's improvisation and willingness to bend rules. In "Darkness on Umbara", Anakin describes Rex as his "first-in-command" and second-to-none in loyalty or capability.

Rex is distinguished by close-shaved blond hair (where most other clones have dark hair) and stylized hawk eyes adorning his helmets; like all members of the 501st, his armor bears blue markings. Later in the series, Rex's armor bears tally marks, representing kills, and Filoni felt it communicated that Rex "is becoming something of a little legend" because he is "a long term survivor of the Clone Wars". He also bears a scar on his chin in homage to actor Harrison Ford, who played Han Solo in the original trilogy.