The Boston Marathon is an annual marathon race hosted by several cities and towns in greater Boston in eastern Massachusetts, United States. It is traditionally held on Patriots' Day, the third Monday of April. Begun in 1897, the event was inspired by the success of the first marathon competition in the 1896 Summer Olympics. The Boston Marathon is the world's oldest annual marathon and ranks as one of the world's best-known road racing events. It is one of six World Marathon Majors. Its course runs from Hopkinton in southern Middlesex County to Copley Square in Boston.

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.