Chloe O'Brian is a fictional character played by actress Mary Lyn Rajskub on the US television series 24. An analyst at CTU Los Angeles (and later New York), she is Jack Bauer's most trusted colleague, often doing unconventional and unauthorised favors for him, even at personal risk to herself. As O'Brian, Rajskub appeared in 125 episodes of 24, more than any other actor except series star Kiefer Sutherland, who appeared in all 192 episodes of the series. UGO.com named her one of the best TV nerds.

She is exceptionally intelligent; in particular she displays extraordinary mastery of computer science. Spending most of her time behind a computer terminal, she is rarely sent on field assignments; however, she has demonstrated proficiency with weapons in Day 4, 5, and 8. She works very well under pressure, yet it is obvious when she is under pressure, and she has always demonstrated a degree of social awkwardness. Despite her lack of social graces, Chloe has gained several friends at CTU and has shown to have a great deal of loyalty to them. She helped CTU Agent Chase Edmunds take care of his daughter from a previous relationship after the child's mother abandoned Chase and their child. She even tried to pass the child off as her own, so as to keep Chase's then-girlfriend, Kim Bauer, from knowing. (Chase did not want Kim to know about his daughter's existence, out of fear that she would leave him.)

She also has a complicated friendship with CTU Agent Jack Bauer. In the beginning, Jack thought of Chloe as an annoyance, though that didn't keep him from using Chloe to engage in unauthorized use of CTU computer resources without their superior's permission. The relationship was one-sided initially, in terms of Jack using Chloe and demanding that Chloe risk her career on her blind faith that Jack was right. Chloe would reluctantly follow Jack's demands and ride out the problems they would make for her with her boss until Jack's requests from her yielded the results that he promised they would.