Randall Flagg is a character created by Stephen King. Flagg has appeared in at least nine of King's novels, either as a major or very minor character. He has a variety of names, usually initialed "R.F." but with occasional exceptions, such as Walter o'Dim (originally envisioned by King as a separate character) in The Dark Tower series. Flagg is described as "an accomplished sorcerer and a devoted servant of the Outer Dark", with supernatural abilities involving necromancy, prophecy, and influence over animal and human behavior. His goals typically center on bringing down civilizations through destruction and conflict.

Flagg first appeared in The Stand as a demonic figure who wreaks havoc after a plague kills most of the population.

Randall Flagg makes his first appearance in King's 1978 apocalyptic novel, The Stand, where he tries to construct a new civilization in the United States after a plague kills most of the population. Flagg is described as a "tall man of no age" in old blue jeans, denim jacket and old cowboy boots. He wears an old Boy Scout knapsack, and his jacket pockets are stuffed with pamphlets from dozens of fringe splinter groups. Flagg’s background is vague, even to him; he says that at some point he just “became”, although he remembers being a Marine, a Klansman, a Viet Cong member and having a hand in the kidnapping of Patty Hearst. In Las Vegas Flagg attracts people drawn to destruction, power and draconian rule, using crucifixion, torture and other punishments on those disloyal to him. His followers reorganize society, rebuilding the city. Flagg plans to attack and destroy a rival emerging civilization (Mother Abagail's Free Zone in Boulder, Colorado) to become the dominant society in the former United States.

After two of Flagg's followers fail to kill the leaders of the Free Zone, the Boulder community sends a group of men to Las Vegas to stop him. The three who reach the city are taken prisoner, and Flagg orders one of them executed for his defiance. As the other two are being prepared for a public execution, one of Flagg's most loyal followers, the Trashcan Man, arrives with a nuclear warhead salvaged from a military base. As Trash dies of radiation poisoning the Hand of God appears, detonating the warhead and annihilating Flagg, his followers, and the prisoners.

An expanded edition of The Stand was published in 1990, restoring text that had been cut from the original edition. It includes an epilogue in which Flagg appears on a beach and finds a primitive tribe ready to fall under his influence.

Another miniseries debuted on CBS All Access in December 2020, and finished airing in February 2021.