The icosathlon, also called double decathlon, is an ultra combined events of track and field competition consisting of 20 events. The word "icosathlon" was formed, in analogy to the word "decathlon", from Greek εἴκοσι (eíkosi, meaning "twenty") and ἄθλος (áthlos, or ἄθλον, áthlon, meaning "contest" or “prize”).

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 icosathlon is contested mainly by male athletes, while female athletes mostly compete in the tetradecathlon. The event is overseen by the International Association for Ultra Multievents (IAUM), which also holds the tetradecathlon based on the women's heptathlon.

The men's world record for the standard icosathlon of 14,571 is held by Joseph Detmer of the United States. The women's world record of 11,653 is held by Lauren Kuntz from the United States.

The vast majority of men's icosathlons are divided into a two-day competition, with the track and field events held in the order below.

Each event is scored according to the decathlon scoring tables or, for non-decathlon events, the World Athletics points tables. At the conclusion of each icosathlon, the competitor with the highest point total is declared the winner.

At major championships, the women's equivalent of the icosathlon is the fourteen-event tetradecathlon. Women's disciplines differ from men's in the same way as for standalone events: the shot, hammer, discus and javelin weigh less, and the sprint hurdles use lower hurdles over 100 m rather than 110 m, over 400m hurdles and 3000m steeple. The points tables used are the same as for the heptathlon and women's decathlon in the shared events. The schedule of events is identical to the men's icosathlon.

One-day icosathlons also exist, with the same program of events succeeding each other more quickly, but respecting the one hour pause of midday between the 3000m steeple and the 110m hurdles.