Brent Matthew Hayden (born October 21, 1983) is a Canadian retired competitive swimmer. Representing Canada for a decade, Hayden is regarded as the fastest swimmer in Canadian history. Hayden won a bronze medal in the 100 m freestyle at the 2012 Summer Olympics in London with a time of 47.80, and was world champion in the same event in 2007 with Filippo Magnini of Italy. By winning the 100 metre, Hayden became the first Canadian in 21 years to win a gold medal at the World Aquatics Championships, and was also the first Canadian to appear in the 100 metre final at the Olympics since Dick Pound at the 1960 Summer Olympics, and the first Canadian to win an Olympic medal in the 100 metre. Hayden added a further three silver and one bronze medal to his World Championship totals.

Hayden is currently the Canadian record-holder in the 200 metre and 100 metre in both the short-course and long-course. He has also held the world record in the 4×100 metre medley relay, and the 4×200 metre freestyle relay. Initially retiring after the London Olympics, Hayden returned to the pool seven years later to compete at the 2020 Summer Olympics in Tokyo.

Hayden's first Olympics were the 2004 Summer Olympics in Athens, as part of the Canadian swimming team. At the time Swimming Canada had been in a steady decline and wound up with no medals in Athens. It was a disappointing first Olympic appearance for Hayden, as inexperience took its toll on the 20-year-old. His best individual finish at the Games was thirteenth in the 200-metre freestyle event. It was further worsened when Hayden was mistaken for a protester and beaten up and arrested by the riot police because he was "tall and wearing a dark shirt" the night before the Closing Ceremonies, a week after his competition ended. The injuries sustained left him unable to compete at the World Short Course Championships which were being held a month later.

In Melbourne for the 2006 Commonwealth Games, Hayden won his first major international silver medal when he finished second in the 50 m freestyle, an event he had not intended on racing. He also added three bronze medals at the same games, two from the relays and one in the 200-metre freestyle final. Those medals were a premonition of the success to come at the 2007 World Aquatics Championships, where he tied for the gold medal with the Italian swimmer in the 100 m freestyle; this was the first World Championship for the Canadians since 1986. In the process of winning the gold, Hayden had also shaved down his own national record in the event down to 48.43 and he also added a bronze from the Worlds in the 4×200-metre freestyle relay. As a result of his victory, the mayor of Mission declared "Brent Hayden Day" on May 26, 2007. As well, the Mission Marlins renamed their swim meet to the "Brent Hayden Invitational" and a large mural, painted on the wall of the Mission Leisure Center, by artist Pascal A. Pelletier, was unveiled.

At the 2008 Summer Olympics in Beijing, he was a member of the Canadian team that finished sixth in the 4 × 100 m freestyle event and fifth in the 4 × 200 m freestyle relay event. Although he qualified third fastest out of sixteen swimmers, one place ahead of Michael Phelps, he did not compete in the semi-finals of the 200 m freestyle. Instead, his coach decided to rest him for the 4×100-metre freestyle, which was held that same day (Hayden set a Canadian record in the opening leg with a time of 47.56). He failed to qualify for the final in the 100 m freestyle despite being the incumbent World Champion in that event. While Hayden was promoted as the best Canadian swimmer at these games with his world title, only fellow long-distance freestyle Ryan Cochrane managed to medal for the nation in the pool, while the remainder of the swim team failed to make the podium despite a general improvement over their 2004 showing.

Hayden attended the 2009 World Aquatics Championships. He failed in the defence of his world title in the 100 m freestyle, placing fourth with a new Canadian record time of 47.27 seconds and missing the podium by 0.02 seconds. César Cielo was the world champion, and set a new world mark of 46.91 seconds. Hayden was the only swimmer in the final to choose not to wear one of the newest "super suits". He chose to wear the Speedo LZR, which was the suit worn by most swimmers at the 2008 Beijing Olympics, whereas every other swimmer in that final was wearing either the new Arena X-Glide or Jaked (both of these suits are completely covered in polyurethane).

Hayden returned to form at the 2010 Commonwealth Games in Delhi, clocking the fastest times in his respective events (50 m freestyle, and 100 m freestyle); setting a Games record in the 100 m freestyle during day one, in the 4 × 100 m freestyle relay final and then breaking his Games record again, in the individual final with a time of 47.98 seconds—0.56 of a second ahead of the silver medallist, Simon Burnett. Hayden held the number one ranking in the world at the time in this event (also being ahead of 22-time Olympic medallist Michael Phelps). In the 50 m freestyle, he won gold with a Games record time of 22.01 seconds. The next season brought further success for Hayden as went on to touch out for the silver on a long glide in the 100 m freestyle final at the 2011 World Aquatics Championships. He had gone into the World Championships having set the previous world's best time the last season. His further successes came as Hayden was working on improving his starting 50 m as he was known more as a finisher in the last 50.

After disappointment at the previous two Olympics when he failed to qualify for the 100 m final, he became the first Canadian to do so in swimming's glamour event since Dick Pound in 1960. He went on to win bronze medal for Canada at the 2012 Summer Olympics in the 100 m freestyle after a strong start placed him in medal contention at the turn. Gold and silver went to American Nathan Adrian and world champion James Magnussen of Australia, while Hayden beat out Yannick Agnel and César Cielo for third; the medals were also presented by Dick Pound. Though only a bronze, the medal was a redemption for Hayden, who was visibly ecstatic at winning an Olympic medal at long last. With his Olympic medal finally in his possession, he said that "There are so many times when you can dream of something but a million out of a million and one times it won't come true. This was one of the things that I was very fortunate as a human being to have happen to me." Hayden announced his retirement from the sport after the 4×100-metre medley relay at the Olympics but said he believed in the Canadian swim team to thrive in the future, saying that "I think in the grand scheme of things, with the people we have, we've done an amazing job … As long as we keep that momentum building, I think we're going to see more success come from more diverse events."