Stephen Joseph Flemmi is an American gangster and convicted murderer and was a close associate of Winter Hill Gang boss Whitey Bulger. Beginning in 1975, Flemmi was a top echelon informant for the Federal Bureau of Investigation (FBI).

The Winter Hill Gang was a loose confederation of organized crime figures in the Boston, Massachusetts, area. It is generally considered an Irish Mob organization, with most gang members and the leadership consisting predominantly of Irish-Americans, though some notable members, such as Johnny Martorano, are of Italian-American descent.