Marvelous Marvin Hagler (born Marvin Nathaniel Hagler; May 23, 1954 – March 13, 2021) was an American professional boxer who competed from 1973 to 1987. He 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 "The Marv" and annoyed that network announcers often did not refer to him as "Marvelous", Hagler legally changed his name to "Marvelous Marvin Hagler" in 1982.

Hagler is an inductee of the International Boxing Hall of Fame and the World Boxing Hall of Fame. He was twice named Fighter of the Year by The Ring magazine and the Boxing Writers Association of America, as well as Fighter of the Decade (1980s) by Boxing Illustrated magazine. In 2001 and 2004, The Ring named him the fourth greatest middleweight of all time and in 2002 named him the 17th greatest fighter of the past 69 years. The International Boxing Research Organization rates Hagler as the sixth greatest middleweight of all time, while BoxRec rates him the 29th greatest boxer of all time, pound for pound. Many analysts and boxing writers consider Hagler to have one of the most durable chins in boxing history, having been knocked down only once during his entire professional career. The lone knockdown, scored by Juan Roldán of Argentina, was debatable.

Hagler was the first child of Robert Sims and Ida Mae Hagler, born on May 23, 1954. His birth year publicly came to light in 1982, when he had to state his date of birth in order to legally change from Marvin Nathaniel Hagler to Marvelous Marvin Hagler. Hagler was raised by his mother in the Central Ward of Newark, New Jersey, United States along with five siblings: sisters Veronica, Cheryl, Genarra, and Noreen, and half brother Robbie Sims, who, like Hagler, would also become a professional boxer. Hagler first put on gloves at the age of ten, for a social worker he knew only as Mister Joe, who taught him sports and got him involved in counseling other children. Hagler dropped out of school at the age of 14 and worked in a toy factory to help support the family. Ida Mae recalled her eldest son had always wanted to box and promised one day to buy her a home. Growing up, Hagler would pretend he was Floyd Patterson or Emile Griffith.

Following the riots of 1967 in which 26 people were killed and $11 million in property damage was caused, including the destruction of the Haglers' tenement, his family moved to Brockton, Massachusetts. Hagler said that looking down on the streets at the looters was almost like watching ants on a picnic table. Ida Mae described the riots as "really terrifying" and nobody left the Hagler apartment for three days. The family lay under Veronica's bed during this time, with a pair of bullets smashing through the bedroom window, and shattering the plaster above the bed. Hagler and his siblings were forbidden from standing up by Ida Mae, who told her children to "stay away from the windows." The family crawled about the five-room apartment, sliding around on cushions to reach the bathroom and kitchen. Once the riot was over, the nearby neighborhoods were mostly in ruin, and many cars had been stripped for parts. After another riot nearly two years later, Hagler and his family got out of Newark and moved to Brockton, Massachusetts.

In 1969, Hagler began boxing after being in a street fight with a local boxer called Dornell Wigfall (Wigfall accosted Hagler at a party and took him outside)—whom he later defeated twice on official fights—with his friends watching, and Wigfall easily beat Hagler and compounded the humiliation by stealing the jacket off Hagler's back. The day after the fight, Hagler determined to become a boxer himself. He first entered Vinnie Vecchione's gym (where, unknown to him, Dornell Wigfall trained), but after one week watching, and of nobody speaking to him, Hagler decided to leave for good. He walked into a gym owned by brothers Pat and Goody Petronelli. They became his trainers and managers. As Hagler needed to be 16 in order to enter some amateur tournaments, he lied about his age, saying that he was born in 1952 instead of 1954. In May 1973, Hagler won the National Amateur Athletic Union (AAU) 165-pound (75 kg) title after defeating Terry Dobbs, a U.S. Marine from Atlanta, Georgia. Ahead of both Aaron Pryor and Leon Spinks, officials also voted him the 'Outstanding Boxer' of the tournament. Hagler subsequently turned professional, finishing his amateur career with a 55–1 record.

National Golden Gloves (Light Middleweight), Lowell, Massachusetts, March 1973:

United States National Championships (Middleweight), Boston, Massachusetts, May 1973:

Hagler was a top-ranked middleweight boxer for many years before he fought for the title. He struggled to find high-profile opponents willing to face him in his early years. Joe Frazier told Hagler, "You have three strikes against you, "You're black, you're a southpaw, and you're good." He often had to travel to his opponents' hometowns to get fights. His first break came when he was offered—on two weeks' notice—a chance against Willie "The Worm" Monroe, who was being trained by Frazier. Hagler lost the decision but the fight was close, so Monroe gave him a rematch. This time Hagler knocked out Monroe in twelve rounds. In a third fight, he defeated Monroe in two rounds.