Mary Sue (or simply Sue) is a usually derogatory expression for a fictional character who is an idealized stand-in for the author and/or reader, or for a story with such a character. A Mary Sue goes beyond a conventional author surrogate. The term originates in fan fiction but is spreading into general use.

Authors of such characters are sometimes referred to as 'Suethors'. Male 'Sues' may be called 'Marty Stus', 'Murray Stus', 'Harry Stus', and/or 'Gary Stus', although the original term is often considered unisex.

Mary Sues are usually original to the author, but some (canon Sues) are created by adapting pre-existing characters. Conversely, original characters, or even self-insertion characters, are not necessarily Mary Sues. Peter Jairus Frigate, for instance, is deliberately based on his author but rarely the focus of the story.