Chloe Anne Sullivan is a fictional character in the television series Smallville, based on the Superman and Superboy comics published by DC Comics. Portrayed by series regular Allison Mack, Chloe was an original character created exclusively for Smallville by series developers Alfred Gough and Miles Millar. Other than main protagonist Clark Kent, Chloe is the only main character to last the whole duration of the show, though Mack signed on for only five episodes in the tenth and final season. The character has also appeared in various literature based on Smallville, a web series, and was later adapted back into the original Superman comics that inspired Smallville.

In Smallville, Chloe is Clark Kent's best friend, Lois Lane's cousin, and the editor of the high school newspaper the Torch; she notices that the meteor rocks (kryptonite) are mutating the citizens of Smallville which she tracks on her "Wall of Weird". She generally teams up with friends Clark and Pete Ross to research and stop meteor-infected people from harming other citizens. In the first five seasons, Chloe harbors an unrequited love for Clark, but eventually accepts her place as his platonic friend and nothing more. In later seasons, Chloe discovers she herself has the metahuman power of empathic healing, though she apparently loses it after an encounter with the alien supervillain Brainiac. In terms of romantic storylines, after Jimmy Olsen (the same-named older brother of Superman supporting character James Bartholomew Olsen) is introduced to the show, he becomes Chloe's boyfriend and later husband, but the pair later divorces after a monster disrupts their wedding. In the show's final two seasons, Chloe finds love with Oliver Queen, otherwise known as the costumed vigilante-archer Green Arrow, whom she eventually marries and has a son with.

Chloe Sullivan has been characterized as independent, intelligent, curious and somewhat impulsive by both the writers and the actress that portrays her. The latter two characteristics often cause Chloe to get into trouble with both her friends and with billionaire industrialists Lionel Luthor and his son Lex, two of the show's primary antagonists. Mack has been recognized with multiple award nominations and wins for her breakout portrayal of Chloe.

Introduced in the series pilot, Chloe spends much of her time helping her best friend Clark Kent (Tom Welling) investigating the mutant citizens of Smallville. These individuals develop special abilities caused by the meteor rocks that fell to Smallville in 1989, and use them to commit crimes. It is established at the start of the first season that Chloe is an aspiring journalist and the editor of the school newspaper The Torch. Her journalistic curiosity—always wanting to "expose falsehoods" and "know the truth"—causes tension with her friends, especially when she is digging into Clark's past in the season two episode "Lineage". In the early seasons, Chloe hides the fact that she is deeply in love with Clark, although the feeling is not reciprocated; she confesses her love to Clark in season two's "Fever" while he is sick and unconscious, but to her dismay he calls out Lana Lang's name in his delirium. Her feelings and frustration for Clark get in the way of her better judgment as she betrays his trust in the season two finale, after witnessing him and Lana Lang (Kristin Kreuk) sharing a kiss in his barn, and agrees to uncover information on Clark for Lionel Luthor (John Glover) in exchange for a job at the Daily Planet.

Chloe and Clark patch their relationship in the season three episode "Whisper", after Clark discovers that she has been helping Lionel. When Chloe stops her investigation, Lionel not only has her blacklisted from the Daily Planet, but also fires her father from LuthorCorp. In season three's "Forsaken", Chloe decides to assist Lex Luthor (Michael Rosenbaum), Lionel's estranged son, with getting Lionel arrested for the past murder of Lex's grandparents; Chloe's hope is to get out from under Lionel's control. In the season three finale, the FBI place Chloe and her father in a safehouse in waiting for Lionel's trial, but the safe-house explodes once Chloe and her father enter and they are presumed dead. This prompts Chloe's maternal cousin Lois Lane (Erica Durance) to come to Smallville to investigate Chloe's death in the fourth season premiere. In season four's "Gone", Clark and Lois team-up and discover that Lex's security team had found the explosives in the safehouse and absconded Chloe and her father to safety before the bomb detonated, and that he has been hiding her ever since. After Chloe's testimony in the same episode, Lionel is convicted of murder and sentenced to life imprisonment. In the season four episode "Pariah", Chloe discovers Clark's secret when Clark's ostracized girlfriend Alicia Baker (Sarah Carter) decides that his secret needs to be exposed to the world in order for him to finally accept who he really is. Alicia expects Chloe to write an exposé about Clark, but Chloe decides that Clark kept his secret for a reason and decides not to write the story instead.

Chloe finally reveals to Clark in the season five premiere that she has known his secret, but that she wanted him to be comfortable enough to tell her on his own. At the same time, Clark reveals that he was not infected by the meteor rocks in Smallville, as Chloe initially suspected, but that he is in fact an alien who was sent to Earth as a baby during the meteor shower of 1989. In season five's "Thirst", Chloe earns her dream job at the Daily Planet, starting in the basement. In the season six episode "Justice", Chloe begins assisting Green Arrow (Justin Hartley) and his team of superheroes under the codename "Watchtower". In "Freak", she discovers she herself is meteor-infected, with an unknown ability, and begins to worry that she is a "time bomb" heading towards insanity. She later discovers in "Progeny" that her institutionalized mother, Moira Sullivan (Lynda Carter), is meteor-infected as well. In the season finale, Chloe learns that her special power lets her heal any wound and even reverse death, when it activates to save Lois. In season seven's "Descent", when Chloe attempts to keep information regarding "The Traveler" a secret from Lex, who is unaware that "The Traveler" is really Clark, he fires her from her job at the Daily Planet. When in "Sleeper", Lana falls into a catatonic state having been attacked by the Kryptonian artificial intelligence known as Brainiac (James Marsters). Chloe takes over Lana's Isis Foundation, a free clinic for individuals who have been infected by the meteor rocks. In the seventh season finale, Chloe is attacked by Brainiac, but her healing powers prevent him from harming her. When she returns home, Jimmy Olsen (Aaron Ashmore), her on-again-off-again boyfriend since season six, proposes marriage. Before Chloe can answer the Department of Domestic Security (DDS) appears and arrests her for hacking into the government database.

At the start of season eight, it is revealed that Chloe was not arrested by DDS, but Lex's security personnel impersonating DDS agents. While subjected to their tests, Chloe discovers that her altercation with Brainiac has apparently caused to her to lose her meteor-related powers, but instilled two new abilities: vast super intelligence and technopathy. Returning to Smallville, Chloe reopens the Isis Foundation. Though she loves Jimmy, she finds herself attracted to paramedic Davis Bloome (Samuel Witwer). In the episode "Abyss", Brainiac's infestation causes Chloe to lose her memories. Clark takes Chloe to his biological father Jor-El, who restores her memories. After Chloe marries Jimmy in "Bride", she is kidnapped by Doomsday, a genetically engineered killing machine bent on destroying Earth and becomes Brainiac's vessel once again. Brainiac attempts to drain the world of all its human knowledge but is stopped and removed from Chloe's body by the Legion, superheroes from the future, in "Legion". In "Hex", Chloe assumes the codename Watchtower full time because she feels her life needs more meaning. Chloe discovers that Davis is Doomsday in "Eternal". She attempts to assist Davis' suicide using kryptonite; when this fails, she stays by his side in order to keep Doomsday under control. In the episode "Beast", she and Davis leave town together; Chloe reasons it will protect Clark. In the season eight finale, she uses black kryptonite to separate Davis from Doomsday; Clark buries Doomsday beneath Metropolis. When Davis discovers that Chloe is still in love with Jimmy, he stabs Jimmy and attempts to kill Chloe; Jimmy impales him on a metal rod, and they both die. Chloe vows to keep the Watchtower Jimmy gave her as a wedding gift open, in the hope that all lost heroes—namely Oliver and his team—will find their way home.

At the start of the ninth season, using Oliver's money, Chloe transforms the Watchtower into an information fortress and superhero headquarters. In this capacity, she acquires a rival in Tess's computer expert Stuart Campbell (Ryan McDonell); her status as superhero information broker also makes her a target for Checkmate bosses Amanda Waller (Pam Grier) and Maxwell Lord (Gil Bellows). Over the course of the season, she grows romantically close to Oliver. In the season ten première, when Oliver is kidnapped by Suicide Squad leader Rick Flag (Ted Whittall), Chloe risks her own sanity by putting on the helmet of Doctor Fate to learn his location. With the information acquired from Fate's helmet, she organizes a switch for Oliver; in Flag's captivity, Chloe fakes suicide and goes off-the-grid. Chloe returns in "Collateral", and reveals that she has been helping Clark, Oliver, and the rest of the heroes while in hiding, having blackmailed the Suicide Squad into helping her. Afterward, she resumes her relationships with the show's protagonists. In the episode "Fortune", Chloe decides to move to Star City to return to journalism following her marriage to Oliver Queen. In a flashforward in the series finale, Chloe is now the mother to a young boy, but remains in touch with Clark and Lois.