A gangster or mobster is a criminal who is a member of a crime organization, such as a gang. The terms are most commonly used in reference to members of the criminal organizations associated with American prohibition and the American offshoot of the Italian Mafia, such as the Chicago Outfit or the Five Families, and individuals such as Al Capone and Bugsy Siegel. Gangsters have been romanticized in popular culture in films such as The Godfather series and TV shows like The Sopranos.

The stereotypical image and myth of the American gangster is most closely associated with the organized crime entities of the Prohibition era of the 1920s and 1930s. In 1920, the Eighteenth Amendment of the United States Constitution banned the sale, manufacture, and transportation of alcohol for consumption. Many gangs sold alcohol illegally for tremendous profit, and used acute violence to stake turf and protect their interest. Often, police officers and politician were paid off or extorted to ensure continued operation.