La Cousine Bette (French pronunciation: ​[la kuzin bɛt], Cousin Bette) is an 1846 novel by French author Honoré de Balzac. Set in mid-19th century Paris, it tells the story of an unmarried middle-aged woman who plots the destruction of her extended family. Bette works with Valérie Marneffe, an unhappily married young lady, to seduce and torment a series of men. One of these is Baron Hector Hulot, husband to Bette's cousin Adeline. He sacrifices his family's fortune and good name to please Valérie, who leaves him for a tradesman named Crevel. The book is part of the Scènes de la vie parisienne section of Balzac's novel sequence La Comédie humaine ("The Human Comedy").In the 1840s, a serial format known as the roman-feuilleton was highly popular in France, and the most acclaimed expression of it was the socialist writing of Eugène Sue. Balzac wanted to challenge Sue's supremacy, and prove himself the most capable feuilleton author in France. Writing quickly and with intense focus, Balzac produced La Cousine Bette, one of his longest novels, in two months. It was published in Le Constitutionnel at the end of 1846, then collected with a companion work, Le Cousin Pons, the following year.

The novel's characters represent polarities of contrasting morality. The vengeful Bette and disingenuous Valérie stand on one side, with the merciful Adeline and her patient daughter Hortense on the other. The patriarch of the Hulot family, meanwhile, is consumed by his own sexual desire. Hortense's husband, the Polish exile Wenceslas Steinbock, represents artistic genius, though he succumbs to uncertainty and lack of motivation. Balzac based the character of Bette in part on his mother and the poet Marceline Desbordes-Valmore. At least one scene involving Baron Hulot was likely based on an event in the life of Balzac's friend, the novelist Victor Hugo.

La Cousine Bette is considered Balzac's last great work. His trademark use of realist detail combines with a panorama of characters returning from earlier novels. Several critics have hailed it as a turning point in the author's career, and others have called it a prototypical naturalist text. It has been compared to William Shakespeare's Othello as well as Leo Tolstoy's War and Peace. The novel explores themes of vice and virtue, as well as the influence of money on French society. Bette's relationship with Valérie is also seen as an important exploration of homoerotic themes. A number of film versions of the story have been produced, including a 1971 BBC mini-series starring Margaret Tyzack and Helen Mirren, and a 1998 feature film with Jessica Lange in the title role.