An Ironman Triathlon is one of a series of long-distance triathlon races organized by the World Triathlon Corporation (WTC), consisting of a 2.4-mile (3.9 km) swim, a 112-mile (180.2 km) bicycle ride and a marathon 26.22-mile (42.2 km) run completed in that order, a total of 140.6 miles (226.3 km). It is widely considered one of the most difficult one-day sporting events in the world.

Most Ironman events have a time limit of 16 or 17 hours to complete the race, course dependent, with the race typically starting at 7:00 am. The mandatory cutoff time to complete the swim is 2 hours and 20 minutes. The mandatory bike cutoff time for when an athlete must have completed their swim, transition, and bike varies generally between 10 hours and 10 hours 30 minutes from when an athlete began their swim. The mandatory run cutoff varies between 16 and 17 hours from when an athlete began their swim. Many races will also have intermediate bike, run, and transition cut-off times specific to each race venue. Any participant who completes the triathlon within these time constraints is designated an Ironman.

The name "Ironman Triathlon" is also associated with the original Ironman triathlon that is now the Ironman World Championship. Held in Kailua-Kona, the world championship has been held annually in Hawaii since 1978 (with an additional race in 1982). Originally taking place in Oʻahu, the race moved to Kailua-Kona in 1981, and it continues today. The Ironman World Championship has become known for its grueling length, harsh race conditions, and television coverage.

Other races are the same distance as an Ironman triathlon but are not produced, owned, or licensed by the WTC. They include The Challenge Family series' Challenge Roth and the Norseman Triathlon.

The event series is owned by The Ironman Group, which is owned by Advance Publications, following the acquisition from the Wanda Sports Group in August 2020.

The idea for the original Ironman Triathlon arose during the awards ceremony for the 1977 Oʻahu Perimeter Relay. Among the participants were representatives of both the Mid-Pacific Road Runners and the Waikiki Swim Club, whose members had long been debating which athletes were more fit, runners or swimmers. On this occasion, U.S. Navy Commander John Collins pointed out that a recent article in Sports Illustrated magazine had declared that Belgian cyclist Eddy Merckx had the highest recorded "oxygen uptake" of any athlete ever measured, so perhaps cyclists were more fit than anyone. Collins and his wife Judy Collins had taken part in the triathlons staged in 1974 and 1975 by the San Diego Track Club in and around Mission Bay, California, as well as the 1975 Optimist Sports Fiesta Triathlon in Coronado, California. A number of the other military athletes in attendance were also familiar with the San Diego races, so they understood the concept when Collins suggested that the debate should be settled through a race combining the three existing long-distance competitions already on the island: the Waikiki Roughwater Swim (2.4 mi or 3.9 km), the Around-Oahu Bike Race (115 mi or 185 km; originally a two-day event) and the Honolulu Marathon (26.219 mi or 42.195 km).

Until that time, no one present had ever done the bike race. Collins figured that by shaving 3 miles (4.8 km) off the course and riding counter-clockwise around the island, the bike leg could start at the finish of the Waikiki Rough Water and end at the Aloha Tower, the traditional start of the Honolulu Marathon. Prior to racing, each athlete received three sheets of paper listing a few rules and a course description. Handwritten on the last page was this exhortation: "Swim 2.4 miles! Bike 112 miles! Run 26.2 miles! Brag for the rest of your life", now a registered trademark.

With a nod to a local runner who was notorious for his demanding workouts, Collins said, "Whoever finishes first, we'll call him the Iron Man." Each of the racers had their own support crew to supply water, food and encouragement during the event. Of the fifteen men to start off in the early morning on February 18, 1978, twelve completed the race. Gordon Haller, a U.S. Navy Communications Specialist, was the first to earn the title Ironman by completing the course with a time of 11 hours, 46 minutes, 58 seconds. The runner-up John Dunbar, a U.S. Navy SEAL, led after the second transition and had a chance to win but ran out of water on the marathon course; his support crew resorted to giving him beer instead.