Konstantin Borisovich "Kostya" Tszyu is a Russian-Australian former professional boxer who competed from 1992 to 2005. He held multiple world championships in the light-welterweight division, including the undisputed and lineal titles between 2001 and 2005. Tszyu was an exceptional all-around boxer-puncher who relied heavily on accuracy, timing, and carried formidable punching power; he is often regarded as one of the hardest-punching light-welterweights in the division's history.

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.