An epistolary novel is a novel written as a series of letters between the fictional characters of a narrative. The term is often extended to cover novels that intersperse documents of other kinds with the letters, most commonly diary entries and newspaper clippings, and sometimes considered to include novels composed of documents even if they do not include letters at all. More recently, epistolaries may include electronic documents such as recordings and radio, blog posts, and e-mails. The word epistolary is derived from Latin from the Greek word epistolē, meaning a letter (see epistle). This type of fiction is also sometimes known by the German term Briefroman or more generally as epistolary fiction.

The Letters of a Portuguese Nun, first published anonymously by Claude Barbin in Paris in 1669, is a work believed by most scholars to be epistolary fiction in the form of five letters written by Gabriel-Joseph de La Vergne, comte de Guilleragues (1628–1684), a minor peer, diplomat, secretary to the Prince of Conti, and friend of Madame de Sévigné, the poet Boileau, and the dramatist Jean Racine.