Donatien Alphonse François, Marquis de Sade (French: [dɔnasjɛ̃ alfɔ̃z fʁɑ̃swa maʁki də ʃɑːˈdeɪ]; 2 June 1740 – 2 December 1814), was a French nobleman, revolutionary politician, philosopher and writer famous for his literary depictions of a libertine sexuality as well as numerous accusations of sex crimes. His works include novels, short stories, plays, dialogues, and political tracts. In his lifetime some of these were published under his own name while others, which Sade denied having written, appeared anonymously.

Sade is best known for his erotic works, which combined philosophical discourse with pornography, depicting sexual fantasies with an emphasis on violence, suffering, anal sex (which he calls sodomy), child rape, crime, and blasphemy against Christianity. Many of the characters in his works are teenagers or adolescents. His work is a depiction of extreme absolute freedom, unrestrained by morality, religion, or law. The words sadism and sadist are derived from his name in reference to the works of fiction he wrote, which portrayed numerous acts of sexual cruelty. While Sade explored a wide range of sexual deviations through his writings, his known behavior includes "only the beating of a housemaid and an orgy with several prostitutes—behavior significantly departing from the clinical definition of sadism". Sade was a proponent of free public brothels paid for by the state: In order both to prevent crimes in society that are motivated by lust and to reduce the desire to oppress others using one’s own power, Sade recommended public brothels where people can satisfy their wishes to command and be obeyed.Despite having no legal charge brought against him, Sade was imprisoned or committed for about 32 years of his life, time divided between facilities such as the Château de Vincennes, the Bastille, and the Charenton asylum, where he died. He wrote many of his works during these periods of confinement. During the French Revolution, he was an elected delegate to the National Convention.

There continues to be a fascination with Sade among scholars and in popular culture. Prolific French intellectuals such as Roland Barthes, Jacques Derrida, and Michel Foucault published studies of him. In contrast, the French hedonist philosopher Michel Onfray has attacked this interest in Sade, writing that "It is intellectually bizarre to make Sade a hero." There have also been numerous film adaptations of his work, including Pasolini's Salò, an adaptation of Sade's controversial book The 120 Days of Sodom, as well as many of the films of Spanish director Jesús Franco.