O Ateneu (in English, The Athenaeum) is a novel written by the Brazilian author Raul Pompeia in 1888, which is considered one of the most prominent examples of Brazilian Naturalism, Impressionism and Realism. The book narrates, in the first person, the story of Sérgio, an eleven-year-old boy who is sent to a well-respected all-male boarding school—known as the Ateneu, hence the book's title—by his father.

The Ateneu has very strict rules imposed by its headmaster—Aristarco—which cause a general sense of riot amongst the students; homosexuality is common amongst most students, which is partially explained due to the presence of just one woman in the school—the nurse and wife of Aristarco, Dona Ema—and the lack of contact with the external world.