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Iridium is a chemical element with the symbol Ir and atomic number 77. It is an extremely dense, hard, brittle, silvery-white transition metal of the platinum group. Iridium is the second densest element known, after osmium. It is one of the rarest elements in the Earth's crust, occurring as a trace element. Iridium is exceptionally resistant to corrosion; it is the only metal that can withstand high temperatures without reacting in air or saltwater. This property makes it valuable for applications where extreme durability is required.

Iridium was discovered in 1803 by Smithson Tennant. Its name is derived from the Greek goddess Iris, who personified the rainbow, because of the striking and varied colors of its salts. Iridium is found in meteorites and is the most abundant platinum-group metal in meteorites. Its presence in terrestrial rocks is much lower, suggesting a significant portion of Earth's iridium originated from extraterrestrial sources, such as asteroid impacts.

The primary uses of iridium are in alloys, particularly with platinum, to improve hardness and strength. These alloys are used in applications such as spark plug electrodes, high-temperature crucibles, and durable pen nibs. Because of its rarity and high cost, its use is generally limited to specialized applications where its unique properties are essential. It is also used in medical devices and in the production of catalytic converters.