The Sin of Harold Diddlebock is a 1947 American comedy film written and directed by Preston Sturges, starring the silent film comic icon Harold Lloyd, and featuring a supporting cast including female protagonist Frances Ramsden, Jimmy Conlin, Raymond Walburn, Rudy Vallee, Arline Judge, Edgar Kennedy, Franklin Pangborn, J. Farrell MacDonald, Robert Dudley, Robert Greig, Lionel Stander and Jackie the Lion. The film's story is a continuation of The Freshman (1925), one of Lloyd's most successful movies.

The Sin of Harold Diddlebock was Sturges' first project after leaving Paramount Pictures, where he had made his most popular films, but the film was not successful in its initial release. It was quickly pulled from distribution by producer Howard Hughes who took almost four years to re-shoot some scenes and re-edit the film, finally re-releasing it in 1950 as Mad Wednesday – but the reception by the general public was no better the second time around.

Lloyd was never to star in another film, turning instead to production, and releasing compilation films featuring his earlier silent film work.