Walter Mitty is a fictional character in James Thurber's short story "The Secret Life of Walter Mitty", first published in The New Yorker on March 18, 1939, and in book form in My World— and Welcome to It in 1942. It was made into a film in 1947.

Mitty is a meek, mild man with a vivid fantasy life: in a few dozen paragraphs he imagines himself a wartime pilot, an emergency-room surgeon, and a devil-may-care killer. The character's name has come into more general use to refer to an ineffectual dreamer, appearing in several dictionaries. The American Heritage Dictionary defines a Walter Mitty as "an ordinary, often ineffectual person who indulges in fantastic daydreams of personal triumphs". The most famous of Thurber's inept male protagonists, the character is considered "the archetype for dreamy, hapless, Thurber Man".

Although the story has humorous elements, some critics see a darker and more significant message underlying the text, leading to a more tragic interpretation of the Mitty character. Even in his heroic daydreams, Mitty does not triumph, several fantasies being interrupted before the final one sees Mitty dying bravely in front of a firing squad. In the brief snatches of reality that punctuate Mitty's fantasies we meet well-meaning but insensitive strangers who inadvertently rob Mitty of some of his remaining dignity.