Sharon van Rouwendaal (NED) is the defending Olympic champion from Rio 2016. With a victory in this event she can become the first athlete to claim multiple Olympic gold medals in marathon swimming. In 2016 she won the first Olympic gold medal for her country in swimming as none were earned for the Netherlands in the 50m pool during the Rio Olympic Games. Van Rouwendaal finished 16th in the women's 200m backstroke on July 30, 2021.

A second Olympic gold for her country was won by Ferry Weertman (NED) who touched first in men's Marathon 10km champion on the next day in Rio 2016. 

Marathon swimmers from the Netherlands have won three of the six gold medals at the Olympic Games, the most from any nation including the Olympic gold earned in the inaugural edition Maarten Van der Weijden (NED) in Beijing 2008. 

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In July 2019 van Rouwendaal finished 10th in the 10km race at the 18th FINA World Championships and earned the final qualification position in Korea. Van Rouwendaal won the women's 10km in three of the last four European Championships. She earned two silver medals at the 2015 Kazan World Championships, one in the Mixed Team Event and her second in the 10km, and another silver medal in the 10km event at the 2017 World Championships in Budapest (HUN).

Finishing a little more than 17 seconds behind the Olympic champion, Rachele Bruni (ITA) earned a silver medal in this event at the Rio 2016 Olympic Games. She took the bronze medal in the women's 10km at the latest FINA World Championships in 2019 and the same medal at the most recent European championships in 2021. Bruni won the World Series in 2015, 2016 and 2019 and placed fifth in the first leg of the FINA Marathon Open Water Swim World Series held in Doha in March 2021.

In the field of 25 women will be five-time World champion Ana Marcela Cunha (BRA) who is hoping to claim her first Olympic medal at 29 years of age. Her best result was a fifth place at the Beijing 2008 Olympic Games, when she was 16 years old. Cunha placed 10th at the Olympic Games in her home country in 2016. The Brazilian has won four World titles in the 25km and one in the 5km since 2010. She was named by FINA as the female open water swimmer of the year a total of six times including 2017, 2018 and 2019. In the Olympic Marathon 10km distance Cunha has claimed three medals at previous World Championships, but none of those is gold. Her highest finish in the Olympic distance was silver at the 2013 FINA World Championships in Barcelona. Cunha was the winner of the first leg of the FINA Marathon Open Water Swim World Series held in Doha in March 2021. 

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Other contenders for the Olympic podium include Xin Xin (CHN) and Haley Anderson (USA). The Chinese World champion won the 10km qualifying event at the 2019 FINA World Championships in Gwangju, Republic of Korea. Xin finished fourth, just off the Olympic podium in 2016 but can become the first Asian marathon swimmer to claim an Olympic medal. Haley Anderson (USA) claimed silver in the women's 10 km at the World Championships in 2019 finishing less than one second behind the Chinese World champion. Anderson is aiming to win her first Olympic gold medal as she won silver in this Marathon 10km at the London 2012 Olympic Games.  

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Only the USA and Germany qualified two swimmers from their nation in the Olympic Marathon due to their top 10 finish in Gwangju in 2019. Ashley Twichell (USA) is Anderson's teammate, finishing 6th in 2019 just 3.3 seconds behind the winner. German swimmers Leonie Beck and Finnia Wunram finished in 8th and 9th place at the 2019 World Championships, both finishing less than 4 seconds behind the gold medallist. Beck placed 4th in the first leg of the FINA Marathon Open Water Swim World Series held in Doha in March 2021.

Cecilia Biagioli (ARG) at age 36 is the oldest competitor in Tokyo, while Anastasia Kirpichnikova (ROC) is the youngest at 21 years of age. Kirpichnikova finished 25th in the 400 freestyle on July 25 and raced in the finals of the 800 and 1500 freestyle events. On July 28 she finished in 7th place in the 1500m free and 8th in the 800m on the last day of July.