Marvelous Marvin Hagler was an American professional boxer. He competed in boxing from 1973 to 1987 and reigned as the undisputed champion of the middleweight division from 1980 to 1987, making twelve successful title defenses, all but one by knockout. Hagler also holds the highest knockout percentage of all undisputed middleweight champions at 78 percent. His undisputed middleweight championship reign of six years and seven months is the second-longest active reign of the 20th century. He holds the record for the sixth longest reign as champion in middleweight history. Nicknamed "Marvelous" and annoyed that network announcers often did not refer to him as such, Hagler legally changed his name to "Marvelous Marvin Hagler" in 1982.

In boxing, the undisputed champion of a weight class is the boxer who simultaneously holds world titles from all recognized major organisations by each other and the International Boxing Hall of Fame. There are currently four major sanctioning bodies: WBA, WBC, IBF and WBO. There were many undisputed champions before the number of major sanctioning bodies recognizing each other increased to four in 2007, but there have only been 19 boxers to hold all four titles simultaneously.