Adolfo Bioy Casares (Spanish pronunciation: [aˈðolfo ˈβjoj kaˈsaɾes]; 15 September 1914 – 8 March 1999) was an Argentine fiction writer, journalist, diarist, and translator. He was a friend and frequent collaborator with his fellow countryman Jorge Luis Borges. He is the author of the fantastique novel The Invention of Morel.

Bioy Casares was born in Buenos Aires, the child of wealthy parents. He began to write in the early Thirties, and his stories appeared in the influential magazine Sur, through which he met his wife, the painter and writer Silvina Ocampo, as well Jorge Luis Borges, who was to become his mentor, friend, and collaborator. In 1940, after writing several novice works, Bioy published the novella The Invention of Morel, the first of his books to satisfy him, and the first in which he hit his characteristic note of uncanny and unexpectedly harrowing humor. Later publications include stories and novels, among them A Plan for Escape, A Dream of Heroes, and Asleep in the Sun. Bioy also collaborated with Borges on an extensive array of works, enhancing his reputation as one of Argentina's most important authors and a key figure in Spanish literature. He was translated into more than sixteen languages and awarded the Premio Cervantes in 1990.