À rebours (French pronunciation: [a ʁəbuʁ]; translated Against Nature or Against the Grain) is an 1884 novel by the French writer Joris-Karl Huysmans. The narrative centers on a single character: Jean des Esseintes, an eccentric, reclusive, ailing aesthete. The last scion of an aristocratic family, Des Esseintes loathes nineteenth-century bourgeois society and tries to retreat into an ideal artistic world of his own creation. The narrative is almost entirely a catalogue of the neurotic Des Esseintes's aesthetic tastes, musings on literature, painting, and religion, and hyperaesthetic sensory experiences.

À rebours contains many themes that became associated with the Symbolist aesthetic. In doing so, it broke from Naturalism and became the ultimate example of "Decadent" literature, inspiring works such as Oscar Wilde's The Picture of Dorian Gray (1890). In his preface for the 1903 publication of the novel, Huysmans wrote that he had the idea to portray a man "soaring upwards into dream, seeking refuge in illusions of extravagant fantasy, living alone, far from his century, among memories of more congenial times, of less base surroundings ... each chapter became the sublimate of a specialism, the refinement of a different art; it became condensed into an essence of jewellery, perfumes, religious and secular literature, of profane music and plain-chant."

À rebours marked a watershed in Huysmans' career. His early works had been Naturalist in style, being realistic depictions of the drudgery and squalor of working- and lower-middle-class life in Paris. However, by the early 1880s, Huysmans regarded this approach to fiction as a dead end. As he wrote in his preface to the 1903 reissue of À rebours:

It was the heyday of Naturalism, but this school, which should have rendered the inestimable service of giving us real characters in precisely described settings, had ended up harping on the same old themes and was treading water. It scarcely admitted—in theory at least—any exceptions to the rule; thus it limited itself to depicting common existence, and struggled, under the pretext of being true to life, to create characters who would be as close as possible to the average run of mankind.

Huysmans decided to keep certain features of the Naturalist style, such as its use of minutely documented realistic detail, but apply them instead to a portrait of an exceptional individual: the protagonist Jean des Esseintes. In a letter of November 1882, Huysmans told Émile Zola, the leader of the Naturalist school of fiction, that he was changing his style of writing and had embarked on a "wild and gloomy fantasy". This "fantasy", originally entitled Seul (Alone), was to become À rebours.

The character of Des Esseintes is partly based on Huysmans himself, and the two share many of the same tastes, although Huysmans, on his modest civil-service salary, was hardly able to indulge them to the same extent as his upper-class hero. Huysmans biographer Robert Baldick names other models for Des Esseintes: Ludwig II of Bavaria, Edmond de Goncourt, Jules Barbey d'Aurevilly, Francis Poictevin, and Robert de Montesquiou.

The most important model, apart from Huysmans himself, was the notorious aristocratic aesthete Montesquiou, who was also the basis for Baron de Charlus in Marcel Proust's À la recherche du temps perdu. Montesquiou's furnishings bear a strong resemblance to those in Des Esseintes's house:

In 1883, to his eternal regret, Montesquiou admitted Stéphane Mallarmé [to his home]. It was late at night when the poet was shown over the house, and the only illumination came from a few scattered candelabra; yet in the flickering light Mallarmé observed that the door-bell was in fact a sacring-bell, that one room was furnished as a monastery cell and another as the cabin of a yacht, and that the third contained a Louis Quinze pulpit, three or four cathedral stalls and a strip of altar railing. He was shown, too, a sled picturesquely placed on a snow-white bearskin, a library of rare books in suitably-coloured bindings and the remains of an unfortunate tortoise whose shell had been coated with gold paint. According to Montesquiou, writing his memoirs many years later, the sight of these marvels left Mallarmé speechless with amazement. 'He went away', records Montesquiou, 'in a state of silent exaltation.... I do not doubt, therefore, that it was in the most admiring, sympathetic, and sincere good faith that he retailed to Huysmans what he had seen during the few moments he spent in Ali Baba's Cave.'