The marathon is a long-distance foot race with a distance of 42.195 kilometres (c. 26 mi 385 yd), 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 worldwide each year, with the vast majority of competitors being recreational athletes, as larger marathons can have tens of thousands of participants.

A creation of the French philologist Michel Bréal inspired by a story from Ancient Greece, the marathon was one of the original modern Olympic events in 1896 in Athens. The distance did not become standardized until 1921. The distance is also included in the World Athletics Championships, which began in 1983. It is the only running road race included in both championship competitions (walking races on the roads are also contested in both).

The name Marathon comes from the legend of Pheidippides, the Greek messenger. The legend states that while he was taking part in the Battle of Marathon, which took place in August or September 490 BC, he witnessed a Persian vessel changing its course towards Athens as the battle was near a victorious end for the Greek army. He interpreted this as an attempt by the defeated Persians to rush into the city to claim a false victory or simply raid, hence claiming their authority over Greek land. It was said that he ran the entire distance to Athens without stopping, discarding his weapons and even clothes to lose as much weight as possible, and burst into the assembly, exclaiming "we have won!", before collapsing and dying.

The account of the run from Marathon to Athens first appeared in Plutarch's On the Glory of Athens in the first century AD, which quoted from Heraclides Ponticus's lost work, giving the runner's name as either Thersipus of Erchius or Eucles. Satirist Lucian of Samosata gave one of the earliest accounts similar to the modern version of the story, but its historical veracity is disputed based on its tongue-in-cheek writing and the runner being referred to as Philippides and not Pheidippides.

There is debate about the historical accuracy of this legend. The Greek historian Herodotus, the main source for the Greco-Persian Wars, mentioned Philippides as the messenger who ran from Athens to Sparta asking for help, and then ran back, a distance of over 240 kilometres (150 mi) each way. In some Herodotus manuscripts, the name of the runner between Athens and Sparta is given as Philippides. Herodotus makes no mention of a messenger sent from Marathon to Athens and relates that the main part of the Athenian army, having fought and won the grueling battle and fearing a naval raid by the Persian fleet against an undefended Athens, marched quickly back from the battle to Athens, arriving the same day.

In 1879, Robert Browning wrote the poem Pheidippides. Browning's poem, his composite story, became part of late 19th-century popular culture and was accepted as a historical legend.

Mount Pentelicus stands between Marathon and Athens, which means that Philippides would have had to run around the mountain, either to the north or to the south. The latter and more obvious route is followed by the modern Marathon-Athens highway (EO83–EO54), which follows the lay of the land southwards from Marathon Bay and along the coast, then takes a gentle but protracted climb westwards towards the eastern approach to Athens, between the foothills of Mounts Hymettus and Penteli, and then gently downhill to Athens proper. As it existed when the Olympics were revived in 1896, this route was approximately 40 kilometres (25 mi) long. It was the approximate distance originally used for marathon races. However, there have been suggestions that Philippides might have followed another route: a westward climb along the eastern and northern slopes of Mount Penteli to the pass of Dionysos, and then a straight southward downhill path to Athens. This route is slightly shorter, 35 kilometres (22 mi), but includes a very steep climb over the first 5 kilometres (3.1 mi).

When the modern Olympics began in 1896, the initiators and organizers were looking for a great popularizing event, recalling the glory of ancient Greece. The idea of a marathon race came from Michel Bréal, who wanted the event to feature in the first modern Olympic Games in 1896 in Athens. This idea was heavily supported by Pierre de Coubertin, the founder of the modern Olympics, as well as by the Greeks. A selection race for the Olympic marathon was held on 22 March 1896 (Gregorian) that was won by Charilaos Vasilakos in 3 hours and 18 minutes. The winner of the first Olympic marathon, on 10 April 1896 (a male-only race), was Spyridon Louis, a Greek water-carrier, in 2 hours 58 minutes and 50 seconds. The marathon of the 2004 Summer Olympics was run on the traditional route from Marathon to Athens, ending at Panathinaiko Stadium, the venue for the 1896 Summer Olympics. That men's marathon was won by Italian Stefano Baldini in 2 hours 10 minutes and 55 seconds, a record time for this route until the non-Olympics Athens Classic Marathon of 2014 when Felix Kandie lowered the course record to 2 hours 10 minutes and 37 seconds.