The decathlon is a combined event in athletics consisting of ten track and field events. The word decathlon is of Greek origin, from δέκα (déka, meaning "ten") and ἄθλος (áthlos, or ἄθλον, áthlon, meaning "feat"). Events are held over two consecutive days and the winners are determined by the combined performance in all. Performance is judged on a points system in each event, not by the position achieved. The decathlon is contested mainly by male athletes, while female athletes typically compete in the heptathlon.

Traditionally, the title of "World's Greatest Athlete" has been given to the person who wins the Olympic decathlon. This began when King Gustav V of Sweden told Jim Thorpe, "You, sir, are the world's greatest athlete" after Thorpe won the decathlon at the Stockholm Olympics in 1912. The current decathlon world record holder is American Ashton Eaton, who scored 9045 points at the 2015 IAAF World Championships.

The event developed from the ancient pentathlon. Pentathlon competitions were held at the ancient Greek Olympics. Pentathlons involved five disciplines – long jump, discus throw, javelin throw, sprint and a wrestling match. Introduced in Olympia during 708 BC, the competition was extremely popular for many centuries. By the sixth century BC, pentathlons had become part of religious games. A ten-event competition known as the "all-around" or "all-round" championship, similar to the modern decathlon, was first contested at the United States amateur championships in 1884 and reached a consistent form by 1890; an all-around was held at the 1904 Summer Olympics, though whether it was an official Olympic event has been disputed. The modern decathlon first appeared on the Olympic athletics program at the 1912 Games in Stockholm.