Donatien Alphonse François, Marquis de Sade (French: [maʁki də sad]; 2 June 1740 – 2 December 1814), was a French nobleman, revolutionary politician, philosopher and writer, famous for his libertine sexuality. 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, criminality, and blasphemy against Christianity. He gained notoriety for putting these fantasies into practice. He claimed to be a proponent of absolute freedom, unrestrained by morality, religion, or law. The words sadism and sadist are derived from his name.

Sade was incarcerated in various prisons and an insane asylum for about 32 years of his life: 11 years in Paris (10 of which were spent in the Bastille), a month in the Conciergerie, two years in a fortress, a year in Madelonnettes Convent, three years in Bicêtre Asylum, a year in Sainte-Pélagie Prison, and 12 years in the Charenton Asylum. During the French Revolution, he was an elected delegate to the National Convention. Many of his works were written in prison.

There continues to be a fascination with Sade among scholars and in popular culture. Prolific French intellectuals such as Roland Barthes, Jacques Lacan, Jacques Derrida and Michel Foucault published studies of him. On the other hand the French hedonist philosopher Michel Onfray has attacked this cult, writing that "It is intellectually bizarre to make Sade a hero." There have also been numerous film adaptions of his work, the most notable being Salò, or the 120 Days of Sodom, an adaptation of his infamous book, The 120 Days of Sodom.