An unreliable narrator is a narrator, whether in literature, film, or theatre, whose credibility has been seriously compromised. The term was coined in 1961 by Wayne C. Booth in The Rhetoric of Fiction. The narrative mode can be developed for several reasons, sometimes to deceive the reader or audience. Most often unreliable narrators are first-person narrators, but sometimes third-person narrators can also be unreliable.

The nature of the narrator is sometimes immediately clear. For instance, a story may open with the narrator making a plainly false or delusional claim or admitting to being severely mentally ill, or the story itself may have a frame in which the narrator appears as a character, with clues to the character's unreliability. A more dramatic use of the device delays the revelation until near the story's end. This twist ending forces readers to reconsider their point of view and experience of the story. In some cases the narrator's unreliability is never fully revealed but only hinted at, leaving readers to wonder how much the narrator should be trusted and how the story should be interpreted.

An exception is an event that did not or could not happen, told within the fictionalized historical novels, speculative fiction, or clearly delineated dream sequences. Narrators describing them are not considered unreliable.