The 2020 London Marathon was the 40th running of the annual marathon race in London, United Kingdom, which took place on 4 October 2020. Due to the COVID-19 pandemic, the race was postponed from 26 April and only allowed elite participants; the mass participation event was cancelled. The event used a different course from usual, consisting of multiple laps around St James's Park.

The men's elite race was won by Ethiopian Shura Kitata, and the women's event was won by Kenyan Brigid Kosgei. The men's wheelchair race was won by Canadian Brent Lakatos, and the women's event was won by Dutchwoman Nikita den Boer.

The 2020 London Marathon was originally scheduled to be held on 26 April but was postponed until 4 October due to the COVID-19 pandemic. It was the first London Marathon to be run in the autumn. On 6 August, it was confirmed that the marathon would go ahead as an elite-only race, with the mass participation event cancelled. Fields of around 30–40 athletes competed for each title. It was the first time that the London Marathon was solely an elite-only event. Due to the COVID-19 pandemic, the race was run without spectators, in a biosecure environment. All athletes were tested for COVID-19 multiple times before the race, and wore face coverings and observed social distancing when not competing. All competitors and event co-ordinators wore a "bump" device which alerted them if they got too close to other people.

As a result of the need for a biosecure environment, the race did not follow its traditional route. Instead, the 2020 London Marathon consisted of 19 laps of length 2.15 kilometres (1.34 mi) around St James's Park, followed by 1,345 metres (0.84 mi) along The Mall, following the finish line of the traditional London Marathon course. The circuits took in The Mall, Horse Guards Parade, Birdcage Walk and Buckingham Palace. The area around St James's Park was closed to prohibit spectators from attending.

The prize money for winners of the 2020 London Marathon was 50 per cent lower than in 2019. For the first time, there was separate prize money awarded to the highest finishing British athletes in the race. The winner's prize money was US$30,000. British competitors who achieved the Olympic qualifying standard at the 2020 London Marathon would have their time count towards the qualification criteria at the delayed 2020 Summer Olympics in Tokyo in 2021. Before both events were postponed, the London Marathon had been scheduled to be used as the British trial event to determine qualification for the 2020 Olympics.

The 2021 London Marathon was postponed from April until October 2021, to maximise the chance of being able to hold a mass participation event.

The women's race included 2019 winner and then marathon world record holder Brigid Kosgei, 2018 winner Vivian Cheruiyot, as well as Ruth Chepng'etich, Roza Dereje, and Valary Jemeli Aiyabei, all of whom had personal best times under 2:20. Ethiopian Degitu Azimeraw was scheduled to race, but withdrew after testing positive for COVID-19.

The favourites for the men's race were 2019 winner Eliud Kipchoge and Kenenisa Bekele, who won the 2019 Berlin Marathon in 2:01:41, two seconds away from Kipchoge's world record at the time. The men's race also featured Mosinet Geremew, Mule Wasihun, Sisay Lemma, and Tamirat Tola, all of whom had personal best times under 2:05. Sondre Nordstad Moen, who broke the European one hour run record earlier in 2020, also competed. Briton Mo Farah, who in September 2020 set the men's world record for the one hour run, acted as a pacemaker for the men's race. Two days before the race, Kenenisa Bekele withdrew from the London Marathon with a calf injury. American Galen Rupp, who won his country's Olympic qualifying event in February 2020, did not compete, after undergoing surgery for an existing injury.