The London Marathon is an annual marathon held in London, England. It is the largest marathon in the world, with over 53,700 finishers in 2024, and 840,318 applicants for the 2025 event. 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 is the current race director and Nick Bitel its chief executive.

The marathon is a long-distance foot race with a distance of 42.195 km, usually run as a road race, but the distance can be covered on trail routes. The marathon can be completed by running or with a run/walk strategy. There are also wheelchair divisions. More than 800 marathons are held throughout the world each year, with the vast majority of competitors being recreational athletes, as larger marathons can have tens of thousands of participants.