The Cigarette Smoking Man (abbreviated CSM or C-Man; sometimes referred to as Cancer Man or the Smoking Man) is a fictional character and one of the primary antagonists of the American science fiction drama television series The X-Files. He serves as the arch-nemesis of FBI Special Agent Fox Mulder. In the show's sixth season, his name is said to be C.G.B. Spender, but Dana Scully suggests this is one of "hundreds of aliases"; the show's characters and fans continue to refer to him by variations of "the Smoking Man" because he is almost always seen chain-smoking Morley cigarettes, and because he was credited in the pilot episode and other episodes (such as the season 1 finale) as "Smoking Man". In the eleventh season, his soliloquy reveals his full birth name to be Carl Gerhard Busch, though whether he has any meaningful "real" identity in his later life outside of the Smoking Man persona is unclear.

Although he utters only four audible words in the entire first season of the show, the Smoking Man eventually develops into the series' primary antagonist. In his early appearances, he is seen in the offices of Section Chief Scott Blevins and Assistant Director Walter Skinner, Mulder and his partner Dana Scully's supervisors. An influential man working for the powers that be, he is a key member in a government-conspiracy unit known only as the Syndicate, who are hiding the truth of alien existence and their plan to colonize Earth. His power and influence remain strong, even after most of the Syndicate is destroyed.

The Smoking Man is portrayed by Canadian actor William B. Davis. When Davis first took the role, the character was written as an extra for the pilot episode. He returned for small cameo appearances during the first season, making increasingly more appearances in the seasons that followed. Davis never received an award for his portrayal alone, but he was nominated for ensemble awards.

TV Guide included him in their 2013 list of The 60 Nastiest Villains of All Time.

The birth date, birthplace, and most of the history of the Smoking Man are never categorically confirmed. One possible version of his past is provided during the fourth-season episode "Musings of a Cigarette Smoking Man", in which the conspiracy theorists known as The Lone Gunmen claim Smoking Man was first documented in Baton Rouge, Louisiana, on August 20, 1940. His father was an ardent communist activist and a spy for the Soviet NKVD who was executed under the Espionage Act of 1917 "before his boy could walk". His mother was a cigarette smoker who died from lung cancer, also when he was an infant. Since he had no surviving family, he became a ward of the state and was sent to various orphanages in the Midwestern United States. He made no friends and spent most of his time reading. This episode contradicts the season 3 episode "Apocrypha" where young Smoking Man was seen in 1953 with Bill Mulder (Fox Mulder's father) as already a shadowy agent and smoking. "Musings of a Cigarette Smoking Man" has the Smoking Man still a young man and not part of the Syndicate in 1963, and he does not smoke yet.

The same episode reveals that by the early 1960s he was a United States Army Special Forces captain involved in black operations and intelligence, and that he served alongside Bill Mulder. The Smoking Man was already involved in the assassinations of Patrice Lumumba and Rafael Trujillo along with the CIA-backed failed invasion of Cuba before being enlisted by a secret cabal to assassinate President John F. Kennedy in Dallas, Texas. The assassination was motivated by Kennedy's mishandling of the Bay of Pigs Invasion and the subsequent Cuban Missile Crisis. As part of the mission, the Smoking Man told the designated patsy, Lee Harvey Oswald, to bring the curtain rods as a way to frame him for the murder. By 1968, the Smoking Man had progressed enough in the hierarchy to push himself for the assassination of Martin Luther King Jr., concerned that King was "talking like a Maoist" and a threat to the American war effort in Vietnam. He did the deed himself because he had "too much respect for the man," and chose to frame a white racist because of his own sympathy for the American civil rights movement.

At the same time, the Smoking Man was an aspiring writer of espionage (and later, science fiction) novels starring a fictional version of himself called Jack Colquitt, based on his own secret activities, but all his drafts were rejected or altered by publishers who found them unrealistic and poorly written. This left him no choice but to continue working for the United States government. By 1991, the Smoking Man's power was pretty much absolute. He was in direct talks with Saddam Hussein and involved in one way or another on the Anita Hill–Clarence Thomas controversy, moving the Rodney King trial to Simi Valley, California, keeping the United States government from interfering in the independence of Bosnia and Herzegovina, the Academy Awards nominations, keeping the Buffalo Bills from ever winning the Super Bowl, drugging the Soviet goalkeeper Vladislav Tretiak before the Miracle on Ice, and executing extraterrestrials that survived landing on American soil.

Nevertheless, X-Files writer Frank Spotnitz stated that this version of events is only one possibility and is not accepted canon.