A total of 51 swimmers, 25 women and 26 men will race in Tokyo.  The top 10 finishers in the women's and men's 10km at the 2019 FINA World Championship were the first athletes qualified for the Olympic Games.  In June of 2021 an additional 15 women and 15 men were added to the field.

Twenty six (26) different federations are represented in the Olympic competition. Germany qualified two men and two women as a result of top 10 finishes in the respective 10km at the 2019 FINA World Championships. France, Italy, and the USA qualified three swimmers while eleven federations are represented by one male and one female. In the first three editions, a total of 28 National Federations (NF's) were represented in Beijing, 33 NF's in London and 29 in Rio.

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Despite being one of the newest disciplines on the Olympic programme, marathon swimming dates back almost 2000 years. The first and oldest "environment" for Marathon or Open Water Swimming took place in the ocean or rivers, as in earlier times pools were not available. In the first three editions of the Olympic Games all swimming events took place in an "open water environment".

•    In Athens 1896, the first modern Olympic Games, three Greek sailors raced for approximately 100m across the Bay of Zea, near Piraeus, with Ioannis Malokinis winning the event. That race was for Greek Naval personnel only but an open competition was also staged and Hungary’s Alfred Hajos took the gold medal in front of approximately 20,000 spectators.

•    At the Paris 1900 Games, swimming events were staged in the River Seine and Great Britain’s John Arthur Jarvis won the 4000m in under an hour.

•    Pontoons were used for the swimming events at the St Louis 1904 Games, giving the open water venue the feel and look of a pool, but it was not until the London Games four years later that an actual swimming pool was first used in the modern Olympic era.

Although FINA added open water events (5km, 10km and 25km) to their competition calendar in 1986, it wasn't until the 1991 FINA World Aquatics Championships held in Perth, Australia that marathon swimming became an official event. At the time, both men's and women's events were held over 25km, often taking over five hours to complete.

The first time the event was held over 10km was at the 9th FINA World Swimming Championships in Fukuoka, Japan in 2001.  It was over this distance that marathon swimming became an official Olympic event less than 10 years later.

The event was first featured on the Olympic programme at the Games of the XXIX Olympiad at Beijing in 2008.  When marathon swimming was first featured as an Olympic event the majority of competitors were those who solely specialised in the sport.

•    The Beijing 2008 winner of the first gold medal for the men's event was Dutch swimmer Maarten van der Weijden, who also overcame a battle against leukemia to become the Olympic champion.

•    Larisa Ilchenko, a Russian long-distance swimmer who had long dominated the World Swimming Championships, won the women's event in Beijing. Ilchenko's late charge to the finish line saw her overhaul her closest rival by a only a few seconds to claim the gold for Russia.

Great Britain won silver and bronze in the women's event at the 2008 Olympic Games in Beijing and a silver in the men's event as well. Since then, all five podiums in marathon swimming have been shared by three different NOCs. Hungary, Russia and Tunisia are the other three NOCs to have claimed gold in marathon swimming at the Olympic Games.

The Netherlands has won three of the six gold medals in marathon swimming at the Olympic Games. They are the only National Olympic Committee (NOC) with multiple Olympic gold medals in marathon swimming. Twelve different NOCs have won Olympic medals in marathon swimming out of a total of 18 awarded medals.

Since the inaugural Olympic competition in Beijing, worldwide participation numbers in the sport swelled following the 2008 Games and the competitive landscape began to change. Swimmers from other long-distance pool events gradually began to enter open water competitions.  

Athletes swim an average of 90km (almost 60 miles) per week, with the majority of their training done in the pool, in addition to land-based strength and conditioning sessions. Acclimatising to water temperatures and conditions is another critical element of a competitor’s preparation for open water events.

The 10km event is often a closely-fought endurance race that is becoming more and more like a sprint event especially at the finish although it takes about two hours for athletes to cover the distance.

David Davies (GBR) and Oussama Mellouli (TUN) and are the only two athletes to win an Olympic medal in both swimming and marathon swimming. Davies stood on the Athens 2004 podium for his Olympic bronze medal in the 1500m freestyle and 4 years later he was on the podium again earning a silver medal in the first edition of the Olympic Swim Marathon 10km in Beijing.

Mellouli is the only swimmer to achieve this at a single Olympic Games. At the Olympic Games London 2012 Mellouli claimed bronze in the men's 1500m freestyle and gold in the men's 10km in marathon swimming.

On the last day of the 2019 FINA World Championships Germany’s Florian Wellbrock offered some truly heroic efforts and established historical milestones when he became the first swimmer winning gold in the open water and in the 1500m freestyle event at the same edition.

For marathon swimming events, officials will review the race video to determine the actual finish order. If the finish order of two or more competitors cannot be determined by video review, the tied competitors will share the same rank.

If swimmers are judged to have committed a minor rule infringement, they are warned by being shown a yellow flag.  For a second infraction, swimmers will be shown a red flag, will be disqualified and must leave the water and take no further part in the event. If, in the opinion of the referee, an action of a swimmer is deemed to be unsporting, the referee may disqualify the swimmer, using a red flag to signal ejection from the race immediately.