Unlike the other swimming events which take place in the more controlled environment of a pool, marathon swimmers compete on an open water 10km course at Odaiba Marine Park.
It is an extremely tactical freestyle race, where athletes conserve as much energy as possible before an exciting sprint finish at the end of the race.
Below, we take a look at the athletes expected to light up the races, the competition schedule, and even some Olympic history in our top things to know about Olympic marathon swimming guide.
Top Olympic marathon swimmers at Tokyo 2020
Germany's Florian Wellbrock will start as favourite to win the men's race, after a superb 2019 that saw him crowned world champion in both the 10km and 1500m events.
But he will take nothing for granted in such a talented open-water field, that also includes French Olympic bronze medallist Marc-Antoine Olivier, who finished 0.2 seconds behind Wellbrock at the world champs.
Dutch reigning Olympic champion Ferry Weertman won't give up his title without a fight, while American former world champ Jordan Wilimovsky and Italy's 1500m freestyle Olympic champion Gregorio Paltrinieri will provide stern competition.
In the women's race, China's Xin Xin will hope to build upon her momentum from the 2019 world champs where she won the gold medal.
Also keep an eye out for Dutch Olympic champion Sharon van Rouwendaal, who will hope to rediscover her golden form in Tokyo.
Olympic marathon swimming schedule at Tokyo 2020
All times are Japan Standard Time (GMT/UTC+9)
4 August 2021
06:30 - 09:10
- Women's 10km open water final
5 August 2021
06:30 - 09:10
- Men's 10km open water final
Olympic marathon swimming venue at Tokyo 2020
The Odaiba Marine Park, which will also host triathlon, offers something very different from most other Olympic venues.
Futuristic structures are replaced by an expanse of natural greenery, coupled with stunning ocean views and a backdrop of Tokyo's cityscape.
During the Tokyo 2020 Games in 2021, a temporary venue that can host 5,500 spectators will be set up in the park.
Olympic marathon swimming competition format at Tokyo 2020
Athletes only have one chance to shine in marathon swimming, with no heats or preliminary rounds.
Instead, there is one single mass-start race, with 25 swimmers in both the men's and women's races.
Olympic marathon swimming history
Marathon swimming is a fairly new Olympic event, having first appeared on the programme at the Beijing 2008 Games.
However, given that the earliest traditional swimming events at the Olympics took place in bodies of open water (as opposed to pools), marathon swimming has revived the use of rivers, sea, and lakes at the Olympics.
This has led to some of the world's most iconic venues ever being used as sports venues in London's Serpentine Lake in Hyde Park, and the coast off Rio de Janeiro's Copacabana beach.
The Netherlands have been the most successful nation at the 10km marathon swim with three gold medals. The first one went to Maarten van der Weijden at Beijing 2008, who overcame leukaemia to become the Olympic champion.
Sharon van Rouwendaal and Ferry Weertman made it a Dutch gold medal-sweep at the Rio 2016 Games.