John Duff, born John McElduff, or John Michael McElduff, because early court records referred to him as John Michael Duff (September 1759 or August 1760 – June 4, 1799 or 1805), was a counterfeiter, criminal gang leader, horse thief, cattle thief, hog thief, salt maker, longhunter, scout, and soldier who assisted in George Rogers Clark's campaign to capture the Illinois country for the American rebel side during the Revolutionary War.