The Shadow is a fictional character created by American magazine publishers Street & Smith and writer Walter B. Gibson. Originally created to be a mysterious radio show narrator and developed into a distinct literary character in 1931 by Gibson, The Shadow has been adapted into other forms of media, including American comic books, comic strips, serials, video games, and at least five feature films. The radio drama included episodes voiced by Orson Welles.

The Shadow debuted on July 31, 1930, as the mysterious narrator of the radio program Detective Story Hour, created to boost sales of Street & Smith's monthly pulp Detective Story Magazine. When listeners of the program began asking at newsstands for copies of "that Shadow detective magazine", Street & Smith launched a magazine based on the character, and hired Gibson to create a concept to fit the name and voice and to write a story featuring him. The first issue of the pulp series The Shadow Magazine went on sale April 1, 1931.

On September 26, 1937, The Shadow, a new radio drama based on the character as created by Gibson for the pulp magazine, premiered with the story "The Death House Rescue", in which The Shadow was characterized as having "the hypnotic power to cloud men's minds so they cannot see him". In the magazine stories, The Shadow did not become literally invisible.

The introductory line from the radio adaptation of The Shadow – "Who knows what evil lurks in the hearts of men? The Shadow knows!" – spoken by actor Frank Readick, has earned a place in the American idiom. These words were accompanied by an ominous laugh and a musical theme, Camille Saint-Saëns' Le Rouet d'Omphale ("Omphale's Spinning Wheel" composed in 1872).

The Shadow, at the end of each episode, reminded listeners, "The weed of crime bears bitter fruit! Crime does not pay...The Shadow knows!"

Some early episodes used the alternate statement, "As you sow evil, so shall you reap evil! Crime does not pay...The Shadow knows!"

To boost the sales of its Detective Story Magazine, Street & Smith Publications hired David Chrisman, of the Ruthrauff & Ryan advertising agency, and writer-director William Sweets to adapt the magazine's stories into a radio series. Chrisman and Sweets thought the upcoming series should be narrated by a mysterious storyteller with a sinister voice and began searching for a suitable name. One of their scriptwriters, Harry Engman Charlot, suggested various possibilities, such as "The Inspector" or "The Sleuth". Charlot then proposed the ideal name for the phantom announcer: "The Shadow".

Thus, beginning on July 31, 1930, "The Shadow" was the name given to the mysterious narrator of the Detective Story Hour radio program. The narrator was initially voiced by James LaCurto, who was replaced after four months by prolific character actor Frank Readick Jr. The episodes were drawn from the Detective Story Magazine issued by Street & Smith, the nation's oldest and largest publisher of pulp magazines at the time. Although the latter company had hoped the radio broadcasts would boost the declining sales of Detective Story Magazine, the result was quite different. Listeners found the sinister announcer much more compelling than the unrelated stories. They soon began asking newsdealers for copies of "that Shadow detective magazine", even though it did not exist.