The Decadent movement (from the French décadence, lit. 'decay') was a late 19th-century artistic and literary movement, centered in Western Europe, that followed an aesthetic ideology of excess and artificiality.

The Decadent movement first flourished in France and then spread throughout Europe and to the United States. The movement was characterized by a belief in the superiority of human fantasy and aesthetic hedonism over logic and the natural world.

The concept of decadence dates to the 18th century, especially from the writings of Montesquieu, the Enlightenment philosopher who suggested that the decline (décadence) of the Roman Empire was in large part due to its moral decay and loss of cultural standards. When Latin scholar Désiré Nisard turned toward French literature, he compared Victor Hugo and Romanticism in general to the Roman decadence, men sacrificing their craft and their cultural values for the sake of pleasure. The trends that he identified, such as an interest in description, a lack of adherence to the conventional rules of literature and art, and a love for extravagant language, were the seeds of the Decadent movement.

The first major development in French decadence appeared when writers Théophile Gautier and Charles Baudelaire used the word proudly to represent a rejection of what they considered banal "progress". Baudelaire referred to himself as decadent in his 1857 edition of Les Fleurs du mal and exalted the Roman decline as a model for modern poets to express their passion. He later used the term decadence to include the subversion of traditional categories in pursuit of full, sensual expression. In his lengthy introduction to Baudelaire in the front of the 1868 Les Fleurs du mal, Gautier at first rejects the application of the term decadent, as meant by the critic, but then works his way to an admission of decadence on Baudelaire's own terms: a preference for what is beautiful and what is exotic, an ease with surrendering to fantasy, and a maturity of skill with manipulating language.

The Belgian Félicien Rops was instrumental in the development of this early stage of the Decadent movement. A friend of Baudelaire, he was a frequent illustrator of Baudelaire's writing, at the request of the author himself. Rops delighted in breaking artistic convention and shocking the public with gruesome, fantastical horror. He was explicitly interested in the Satanic, and he frequently sought to portray the double-threat of Satan and Woman. At times, his only goal was the portrayal of a woman he'd observed debasing herself in the pursuit of her own pleasure. It has been suggested that, no matter how horrific and perverse his images could be, Rops' invocation of supernatural elements was sufficient to keep Baudelaire situated in a spiritually aware universe that maintained a cynical kind of hope, even if the poetry "requires a strong stomach". Their work was the worship of beauty disguised as the worship of evil. For both of them, mortality and all manner of corruptions were always on their mind. The ability of Rops to see and portray the same world as they did made him a popular illustrator for other Decadent authors.

The concept of decadence lingered after that, but it was not until 1884 that Maurice Barrès referred to a particular group of writers as Decadents. He defined this group as those who had been influenced heavily by Baudelaire, though they were also influenced by Gothic novels and the poetry and fiction of Edgar Allan Poe. Many were associated with Symbolism, others with Aestheticism. The pursuit of these authors, according to Arthur Symons, was "a desperate endeavor to give sensation, to flash the impression of the moment, to preserve the very heat and motion of life", and their achievement, as he saw it, was "to be a disembodied voice, and yet the voice of a human soul".

In his 1884 Decadent novel À rebours (English: Against Nature or Against the Grain), Joris-Karl Huysmans identified likely candidates for the core of the Decadent movement, which he seemed to view Baudelaire as sitting above Paul Verlaine, Tristan Corbière, Theodore Hannon and Stéphane Mallarmé. His character Des Esseintes hailed these writers for their creativity and their craftsmanship, suggesting that they filled him with "insidious delight" as they used a "secret language" to explore "twisted and precious ideas".

Not only did À rebours define an ideology and a literature, but it also created an influential perspective on visual art. The character of Des Esseintes explicitly heralded the paintings of Gustave Moreau, the 17th-century Dutch engraver Jan Luyken's illustrations to the Martyrs Mirror and the lithographs of Rodolphe Bresdin and Odilon Redon. The choice of these works established a decadent perspective on art which favored madness and irrationality, graphic violence, frank pessimism about cultural institutions, and a disregard for visual logic of the natural world. It has been suggested that a dream vision that Des Esseintes describes is based on the series of satanic encounters painted by Félicien Rops.