thumb|right|[[Ernst van Dyk has won the Boston Marathon 10 times, more than any other athlete.|alt=Ernst van Dyk]]
The Boston Marathon, one of six World Marathon Majors, is a race which has been held in the Greater Boston area in Massachusetts since 1897, making it the oldest annual marathon in the world.
The event is held on Patriots' Day, which was April 19 (or April 20 if April 19 was a Sunday) until the implementation of the Uniform Monday Holiday Act in 1971. Since 1971, except in 2020 (race not held) or 2021 (Columbus Day holiday), the Boston Marathon is held on the third Monday in April. Various factors meant that until 1957 the course varied in length, due to which the marathon recognizes several course records that are slower than previous records due to being run on longer courses. The first Boston Marathon included only 15 runners, all of whom were men, and was won by John McDermott.
Through the 2025 edition: overall winners (men's open division) have represented 19 countries, with Americans having won the marathon the most, doing so on 43 occasions; Kenyans have won 26 times; and Canadians 16 times. Winners in the women's open division have represented 10 countries: Kenyan women have won 17 races, American women have won 16 times, and Ethiopian women have won 8 times. Ernst van Dyk is the most successful individual athlete, having won the men's wheelchair division 10 times. Course records are held by John Korir (men), Sharon Lokedi (women), Marcel Hug (men's wheelchair), and Manuela Schär (women's wheelchair).
Clarence DeMar won the men's open race seven times, more than any other runner, achieving his first victory in 1911 and his last in 1930. Women were only officially allowed to run the race beginning in 1972, though female runners had unofficially participated beginning in 1966 despite breaching the rules of the Amateur Athletic Union. while Nina Kuscsik was the first official winner in 1972. The first female wheelchair finisher, Sharon Rahn, came in 1977. Ernst van Dyk's ten wins in the men's wheelchair division are the most of any athlete at Boston, while Jean Driscoll leads the women's wheelchair division with seven wins, and holds the overall record for the most consecutive victories, also seven. Tom Davis has won the first three men's handcyclist races since it was officially recognized in 2017, and still holds the course record. Alicia Dana has won the women's handcycle race three times, setting a course record each time.
The course was designed to replicate the original marathon in Greece; a hilly point-to-point race, and as such has not been the venue for many world records.