Sheldon Lee Cooper, B.S., M.S., M.A., Ph.D., Sc.D., is a fictional character and one of the protagonists in the 2007–2019 CBS television series The Big Bang Theory and its 2017–2024 spinoff series Young Sheldon, portrayed by actors Jim Parsons and Iain Armitage respectively (with Parsons as the latter series' narrator). For his portrayal, Parsons won four Primetime Emmy Awards, a Golden Globe Award, a TCA Award, and two Critics' Choice Television Awards. The character's childhood is the focus of Young Sheldon, in which he grows up as a child prodigy in East Texas with his family: twin sister Missy Cooper, father George Cooper Sr., brother George Cooper Jr., mother Mary Cooper, and his grandmother, Connie Tucker.

The adult Sheldon is a senior theoretical physicist at the California Institute of Technology (Caltech), and for the first ten seasons of The Big Bang Theory shares an apartment with his colleague and best friend, Leonard Hofstadter (Johnny Galecki); they are also friends and coworkers with Howard Wolowitz (Simon Helberg) and Rajesh Koothrappali (Kunal Nayyar). In season 10, Sheldon moves across the hall with his girlfriend Amy Farrah Fowler (Mayim Bialik), in the former apartment of Leonard's wife Penny (Kaley Cuoco).

He has a genius-level IQ of 187; however, he displays a fundamental lack of social skills, a tenuous understanding of humor, and difficulty recognizing irony and sarcasm in other people, although he himself often employs them. The antihero of the series, he exhibits highly idiosyncratic behaviour and a general lack of humility, empathy, and toleration. These characteristics provide the majority of the humor involving him, which are credited with making him the show's breakout character. Some viewers have asserted that Sheldon's personality is consistent with autism spectrum disorder (or what used to be classified as Asperger's Syndrome). Co-creator Bill Prady has stated that Sheldon's character was neither conceived nor developed with regard to Asperger's, although Parsons has said that in his opinion, Sheldon "couldn't display more facets" of Asperger's syndrome.

The character of Sheldon Cooper was inspired by a computer programmer personally known to series co-creator Bill Prady. He and his friend Leonard Hofstadter are named in honor of actor/producer Sheldon Leonard, scientist Robert Hofstadter, and Nobel Prize Laureate Leon Cooper. Chuck Lorre originally intended Johnny Galecki to play the role, but Galecki thought he would be "better suited" for the character of Leonard. Lorre said that when Jim Parsons auditioned for the role, he was "so startlingly good" that he was asked to re-audition "to make sure he hadn't gotten lucky".

Sheldon and his fraternal twin sister, Missy, were born on February 26, 1980, at a Kmart in Galveston, Texas, and raised in Medford, a fictional small town in East Texas that is a three-hour drive from Dallas, along with their older brother, George Jr., by their mother, Mary Cooper, an overtly devout Baptist, and their father, George Cooper Sr., a football coach. His first word was 'hypotenuse'; he said this at four months old. Sheldon once got his father fired when he told Mr. Hinckley, a store owner, that George was stealing from the cash register. In Young Sheldon, this is retconned: his father is a football coach who was fired from his coaching position in Galveston because he disclosed that other coaches were illegally recruiting players to their school, forcing the family to return to Medford. He does drink, mostly beer, and is a loving father who is trying to understand his intellectually gifted son. The only member of his family to have actively encouraged his work in science was his maternal grandfather, whom he cherished and affectionately called "Pop-Pop", and who died when Sheldon was five years old. Pop-Pop's loss is what caused Sheldon to not like Christmas very much when his Christmas wish to bring Pop-Pop back did not come true. Sheldon's closest relative is his maternal grandmother whom he affectionately calls "Meemaw", and who in turn calls him "Moon Pie". His aunt was also said to have encouraged his work in science by giving him medical equipment, "in case his work in physics failed, he'd have a 'trade' to fall back on". In Young Sheldon, it is shown that his childhood friend Tam was the one who introduced him to non-scientific interests such as comic books and Dungeons & Dragons.

Sheldon was interested in science from an early age, and was a child prodigy, although due to his behavioral quirks and his lack of humility about his superior intellect, he was bullied by classmates and neighbors. Sheldon entered college at the age of eleven, and at age fourteen he graduated from college summa cum laude. Throughout the entire run of The Big Bang Theory, the college he went to for undergraduate studies had not been revealed, but he always felt it was superior to his fellow Big Bang Theory colleagues' alma maters of Princeton (Leonard), Cambridge (Raj), MIT (Howard) and Harvard (Amy); however, it is implied in "The Tam Turbulence" (Season 12, Episode 4) that Sheldon is in fact a Caltech alumnus, and in Young Sheldon he is said to enroll at the fictional East Texas Tech before finally enrolling at Caltech in the final episodes of the series. From then, he worked on his doctorate, was a visiting professor at the University of Heidelberg in Germany, was the youngest person at the time (fourteen and a half) to receive the Stevenson Award, and has appeared on the cover of Journal of Physical Chemistry A. Sheldon is now a theoretical physicist doing research at Caltech, although he stated in Young Sheldon that he could not see himself living in California due to their carefree lifestyles.

Like Leonard, Raj, and Howard, Sheldon is characterized as being highly intelligent, but he tends to display childish qualities, such as extreme stubbornness and meanness. It is claimed by Bernadette that the reason Sheldon is sometimes mean is because the part of his brain that tells him it is wrong to be mean is "getting a wedgie from the rest of his brain". However, in season 8's "The Space Probe Disintegration", Sheldon tearfully admits to Leonard that he is aware of how his behavior comes across. The first four episodes of The Big Bang Theory portray Sheldon slightly inconsistently with respect to his later characterization: according to Prady, the character "began to evolve after episode five or so and became his own thing".

Sheldon frequently states that he possesses an eidetic memory (although his powers of autobiographical recall are more like hyperthymesia) and an IQ of 187, although he claims his IQ cannot be accurately measured by normal tests. He originally claimed to have a master's degree and two doctoral degrees, but this list has increased. Sheldon possesses a mastery (and extensive knowledge) of various subjects such as physics, spectroscopy, radiology, chemistry, pharmacology, both anatomy and physiology, zoology, microbiology, astronomy, cosmology, algebra, geometry, calculus, trigonometry, economics, computers, software engineering, robotics/cybernetics, artificial intelligence, nanotechnology, history, anthropology, geography, linguistics, cryptography, forensics, vexillology (he hosts a webshow called Fun With Flags) and railroad engineering (he is a well-known railfan and a fancier of model trains), and in addition to being fluent in Klingon, has some knowledge of Swedish, Finnish, Mandarin, and Hindi. It is also revealed in Young Sheldon that he is proficient in Spanish, although that particular proficiency has apparently decreased by the time of The Big Bang Theory. He also shows great musical talent, knowing how to play the piano (as does Parsons in real life) and the theremin (which Parsons had learned to play in real life) and can sing with perfect pitch. Although his friends have similar intellects to him, his egocentrism and stubbornness frequently frustrate them. Sheldon occasionally uses slang (in a very unnatural fashion) and follows jokes with his catchphrase "Bazinga!", which is now an officially registered trademark of Warner Bros. He is uncomfortable with human physical contact and has mysophobia, which makes his exceptionally rare hugs extremely awkward and painful-looking. He also has hemophobia and synesthesia, the latter being demonstrated when he described prime numbers as being red, twin primes as being pink and smelling like gasoline and Fudgesicles as tasting like the speed of light. Sheldon has difficulty coping when asked to keep a secret, when he is interrupted, or when he hears arguing. He is also a notary public and uses his knowledge in law and contracts usually for his own advantage and is always distressed when challenged in a legal aspect that he cannot logically defend. In his mannerisms, Sheldon also shows symptoms associated with obsessive-compulsive personality disorder, as is suggested within the show itself by Amy in regard to how, whenever approaching the door of an occupied room—say the bathroom—he must knock three times, then say the person's name, and must repeat this three times; this was revealed as a result of him seemingly walking in on his father with another woman (although in reality it was just his mother roleplaying) when he was thirteen ("The first [knock] is traditional, but 'two' and 'three' are for people to get their pants on."). Upon entering a person's home, he must select the proper seat before sitting down. When it was suggested by Penny that he "just sit anywhere", his response is "Oh, no, if only it were that simple!" Because of his rigidity and stubbornness, only his mother and Bernadette, both possessing strong maternal personalities, have shown the ability to order him to do things. Like his friends, Sheldon is fond of comic books (mostly from the DC Universe), costumes, video games (he also mentions his ownership of various vintage games and systems), roleplaying games, tabletop games, collectible card games, and action figures. Sheldon has restraining orders from his heroes Leonard Nimoy, Carl Sagan, and Stan Lee, as well as television scientist Bill Nye. Sheldon often wears vintage T-shirts adorned with superhero logos. One of his shirts is emblazoned with the number 73, and in the episode "The Alien Parasite Hypothesis" he explains that 73 is his favourite number because it is the 21st prime number, its mirror, 37, is the 12th prime number and its mirror (21) is the product of multiplying 7 and 3, also in binary, 73 is 1001001, a palindrome.