Wladimir Klitschko (born 25 March 1976) is a Ukrainian former professional boxer who competed from 1996 to 2017. He held multiple heavyweight world championships between 2000 and 2015, including unified titles between 2008 and 2015. During this time he also held the International Boxing Organization (IBO) and Ring magazine titles.

A strategic and intelligent boxer, Klitschko is considered to be one of the greatest heavyweight champions of all time. He was known for his exceptional knockout power, using a strong jab; straight right hand; and left hook, quick hand speed, formidable physical strength (which he often employed when clinching opponents), and his athletic footwork and mobility - unusual for boxers of his size.

As an amateur, Klitschko represented Ukraine at the 1996 Olympics, winning a gold medal in the super-heavyweight division and ending his amateur run with 134 wins and 6 losses. After turning professional later that year, he defeated Chris Byrd in 2000 to win the World Boxing Organization (WBO) heavyweight title. Klitschko's first reign as champion ended in an upset knockout loss to Corrie Sanders in 2003, which was followed by another upset knockout loss to Lamon Brewster in 2004. It was during this time that Klitschko hired Emanuel Steward as his trainer, which began an eight-year partnership that lasted until Steward's death in 2012. In particular, Steward was credited with Klitschko's transition from an aggressive puncher to a more defensively-oriented boxer, much as he had done with Lennox Lewis from 1995 to 2003.

From 2004 to 2015, Wladimir and his brother Vitali Klitschko (himself a multiple-time world champion) dominated heavyweight boxing, a period typically known as the "Klitschko Era" of the division. In 2006, Wladimir regained a portion of the world heavyweight championship after defeating Byrd in a rematch to win the International Boxing Federation (IBF) and IBO titles. He won his second WBO title by defeating Sultan Ibragimov in 2008. Following his defeat of Ruslan Chagaev in 2009, Klitschko was awarded the Ring title, and lastly he won the World Boxing Association (WBA) title (Unified version, later Super version) from David Haye in 2011.

During Klitschko's reign as world heavyweight champion, his fights would reportedly generate up to 500 million viewers worldwide. Klitschko holds all-time records for the longest time spent as world heavyweight champion at 4,382 days and most fighters beaten for the world heavyweight championship, at 23. He also holds records for the most wins and title defences of the unified championship in professional boxing history. In 2011, both Wladimir and Vitali entered the Guinness World Records book as brothers with most world heavyweight title fight wins (30 at the time; 40 as of 2020). Klitschko was inducted into the International Boxing Hall of Fame Class of 2021, having been elected in his first year of eligibility.

Klitschko was born in Semipalatinsk, Kazakh SSR, Soviet Union (now Semey, Kazakhstan). He is the son of Nadiia Ulianivna Klychko (née Bulyno) and Volodymyr Rodionovych Klychko (1947–2011), a Soviet Air Force major general and a military attaché of USSR in East Germany. He is the younger brother of former WBC, WBO, and Ring magazine heavyweight champion Vitali Klitschko, the current Mayor of Kyiv. Klitschko's paternal grandmother was Jewish.

The Klitschko brothers lived as children in Czechoslovakia from 1980 to 1985, where their father was stationed with the Soviet Forces. They attended a school designated for children of Soviet soldiers in the town of Mimoň in Northern Bohemia.

The Klitschko brothers lived in Pripyat, Ukrainian SSR from 1985 to 1986, when the town was evacuated following the Chernobyl nuclear disaster. The elder Klitschko was also one of the commanders in charge of cleaning up the effects of the Chernobyl nuclear disaster in 1986 and was afterward diagnosed with cancer. He also served as a military attache at the embassy of Ukraine in Germany. His mother is Nadiya Ulyanivna.