Konstantin Borisovich "Kostya" Tszyu (/ˈkɒstə ˈzuː/; Russian: Константин Борисович «Костя» Цзю, ; born 19 September 1969) is a Soviet-born Australian former professional boxer who competed from 1992 to 2005. He held multiple light-welterweight world championships, including the undisputed and lineal titles between 2001 and 2005. Tszyu was an exceptional all-around boxer-puncher who relied heavily on accuracy and timing, and carried formidable punching power; he is often regarded as one of the hardest-punching light-welterweights in the division's history.
As an amateur, Tszyu represented the Soviet Union, winning a bronze medal in the lightweight division at the 1989 World Championships, and gold in the light-welterweight division at the 1991 World Championships. He also won consecutive gold medals at the 1989 and 1991 European Championships.
In 1995, Tszyu won his first of many professional world championships at light-welterweight—the IBF title—by defeating Jake Rodríguez. Five successful defences made until an upset loss to Vince Phillips in 1997, which was Tszyu's first professional defeat. He would spend the next eight years undefeated, winning the vacant WBC title in 1999 and the WBA (Super) title in 2001. Tszyu scored arguably his most famous win that year, when he knocked out Zab Judah to reclaim the IBF title, as well as the vacant Ring magazine and lineal titles, thus becoming the first to undisputed light-welterweight champion in over 30 years. In 2005, in what would be his final fight, Tszyu was stopped by Ricky Hatton.