The London Marathon (also known as the TCS London Marathon for sponsorship reasons) is an annual marathon held in London, England. Founded by athletes Chris Brasher and John Disley in 1981, it is typically held in April, although it moved to October for 2020, 2021, and 2022 due to the COVID-19 pandemic. The largely flat course is set around the River Thames, starting in Blackheath and finishing at The Mall. Hugh Brasher (son of Chris) is the current race director and Nick Bitel its chief executive.

The race has several components: it has a mass race for the public, professional races for men and women long-distance runners, elite level wheelchair races for men and women, plus a 3-mile mini marathon event for under-17 athletes. There is a significant charity running aspect to the marathon, with participants helping to raise over £1 billion since its founding, including £67 million at the 2024 London Marathon which was the highest amount for a single-day fund-raising event.

Since 2006, the elite race has been part of the World Marathon Majors, which includes seven of the world's top level marathon races. The London Marathon has seen the marathon world record broken on seven occasions: Khalid Khannouchi broke the men's record in 2002, while women's records have been broken by Grete Waitz (1983), Ingrid Kristiansen (1985), Paula Radcliffe (2002, 2003, 2005) and Mary Jepkosgei Keitany (2017). The current elite course records are held by Kelvin Kiptum (2:01:25 in 2023) and Paula Radcliffe (2:15:25 in 2003). The current wheelchair course records are held by Marcel Hug (1:23:44 in 2023) and Catherine Debrunner (1:38:24 in 2022). The race often has a title sponsorship, it has been branded the "TCS London Marathon" since 2022.

NOTE: The 2020 race was restricted to elite athletes in able-bodied (30 for each sex) and invited wheelchair athletes.

The London Marathon was not the first long-distance running event held in the city, which has a long history of marathon events. The Polytechnic Marathon (also known as the Poly) was first held in 1909.

The current London Marathon was founded in 1981 by Olympic champion and journalist Chris Brasher and athlete John Disley. Shortly after completing the New York City Marathon in November 1979 Brasher wrote an article for The Observer newspaper which began:

To believe this story you must believe that the human race be one joyous family, working together, laughing together, achieving the impossible. Last Sunday, in one of the most trouble-stricken cities in the world, 11,532 men and women from 40 countries in the world, assisted by over a million black, white and yellow people, laughed, cheered and suffered during the greatest folk festival the world has seen.

The first London Marathon was held on 29 March 1981, more than 20,000 applied to run. 6,747 were accepted and 6,255 crossed the finish line on Constitution Hill. The Marathon's popularity has steadily grown since then. As at 2009, 746,635 people have completed the race since its inception. In 2010, 36,549 people crossed the line, the biggest field since the race began. The first wheelchair marathon race was held in 1983 and the event was credited with reducing the stigma surrounding disabled athletes. In 2013 the IPC Athletics Marathon World Cup was held within the London Marathon featuring athletes of both genders in the T42–T46 and T11–T13 categories. In August 2013 it was announced that the event would be staged in London until 2017 and feature athletes in the T11-T12, T13, T42-T44, T43, T45-46, T51-52 and the T53-54 class.