Sun Yang (Chinese: 孙杨; Mandarin pronunciation: [swə́n.jǎŋ]; born 1 December 1991) is a Chinese Olympic and former world-record-holding competitive swimmer. In 2012, Sun became the first Chinese athlete to win an Olympic gold medal in men's swimming. Sun is the first male swimmer in history to earn Olympic and World Championship gold medals at every freestyle distance from 200 to 1500 metres. A three-time Olympic gold medalist and eleven-time world champion, he is the most decorated Chinese swimmer in history. In 2017, NBC Sports described him as "very arguably the greatest freestyle swimmer of all time".

In September 2018 Sun was involved in a controversial incident during an out-of-competition doping test at his home. The doping control assistant (DCA) lacked proper accreditation and took photos of Sun without his permission, leading him to refuse the blood sample collection. A security guard broke open the container to prevent the testers from taking the vials. The DCA later admitted to unprofessional conduct. FINA ruled that Sun did not violate anti-doping rules due to the DCA's actions. However, on 28 February 2020, the Court of Arbitration for Sport (CAS) overturned FINA's decision, banning Sun for eight years for tampering with the doping control process. Some analysts opined that the trial was marred by severe translation problems, conflict of interest, bias and an "absence of evidence" around doping activity. Even so, CAS found no evidence of doping, so Sun retained his medals. On 22 June 2021, after a retrial, CAS upheld the ban but reduced it to four years and three months due to issues related to lack of impartiality in the prior trial.

Sun was born on 1 December 1991 in Hangzhou, Zhejiang, China. He was named after his parents' surnames. Sun is an only child, and his family is athletic. His father, Sun Quanhong (孙全洪), was a basketball player for Anhui Tigong. His mother, Yang Ming (杨明), was a volleyball player for Zhejiang and is a sports coach. Sun started swimming in 1998, and his potential was recognised by a teacher at Jinglun Sports School in Hangzhou.

Sun made his international debut at the 2007 World Championships in Melbourne, Australia. He finished 32nd in the 400 metre freestyle and 23rd in the 800 metre freestyle. In early 2008, he competed at the "Good Luck Beijing" China Open, a test event for the 2008 Summer Olympics. He finished 2nd in the 400 metre freestyle with a time of 3:49.34. At the Olympics, Sun, aged 16, finished 28th in the 400 metre freestyle, unable to reach the final. He later came 7th in the qualifying heats of the 1500 metre freestyle, reaching the final, where he finished last. Sun rebounded the following year at the 2009 World Championships, where he won his first international medal, a bronze, in the 1500 metre freestyle with a time of 14:46.84. He finished 18th in the 400 metre freestyle with a time of 3:47.51.

Sun achieved further success during the 2010 Asian Games. He won gold medals in the 1500 metre freestyle and the 4 × 200 metre freestyle relay, and silver medals in the 200 metre and 400 metre freestyle. His 1500-metre freestyle time of 14:35.43 was a new Asian record and the second-fastest time in history, behind Grant Hackett, whose record Sun would surpass the following year. Sun was subsequently named Rookie of the Year at the 2010 China Central Television (CCTV) Sports Awards.

On the first day of competition at the 2011 World Championships in Shanghai, Sun came 2nd in the 400 metre freestyle with a time of 3:43.24, 1.2 seconds behind South Korean Park Tae-hwan. Three days later, Sun won his first World Championship title in the 800 metre freestyle with a time of 7:38.57. After a day of rest, he swam the anchor leg in the 4 × 200 metre freestyle relay, helping the Chinese team win bronze. The last day of competition, Sun competed in the 1500 metre freestyle. He won the race with a time of 14:34.14, ten seconds ahead of the rest of the competition. At age 19, Sun beat Hackett's world record of 14:34.56, which had stood for ten years. This was the longest-held world record in swimming and the only men's swimming world record not to have been beaten during the techsuit era.

For the 2012 Summer Olympics, Sun was considered a favourite in the 400 metre and 1500 metre freestyle, as well as a contender in the 200 metre freestyle. In his first event, the 400 metre freestyle, he won the gold medal with a time of 3:40.14, breaking the Olympic record previously held by Ian Thorpe and becoming the first Chinese male swimmer to win a gold medal at the Olympics. His time was also the third fastest in history, 0.07 seconds away from the world record, and setting a new Asian record. After a day of recuperating, Sun competed in the 200 metre freestyle. He won a silver medal in the final, tying with Park Tae-hwan with a time of 1:44.93, a new national record. The following day, Sun swam the anchor leg in the 4 × 200 metre freestyle relay to help the Chinese team win bronze, China's first Olympic medal in a men's relay event.

After the relay, Sun did not return to the pool until the last day of competition to compete in his strongest event, the 1500 metre freestyle. He had qualified for the final with the fastest time. At the initial line-up on the starting blocks, a step-down command was issued because of noise from the crowd, but instead of stepping back, Sun reacted by diving in. The officials ruled the early dive was the result of noise from the crowd and did not constitute a false start, and Sun was allowed to continue in the competition. He went on to win the race with a time of 14:31.02, breaking his own world record by over three seconds. He finished over eight seconds ahead of second-place finisher, Canadian Ryan Cochrane. With two gold medals and a team bronze medal, Sun became the most decorated Chinese male swimmer in Olympic history, and the first swimmer to win the 400/1500 double since Vladimir Salnikov at the 1980 Moscow Olympics. Sun completed the last 50 metres of the race in 25.68 seconds, nearly two seconds faster than Cochrane.