The rainbow trout (Oncorhynchus mykiss) is a species of salmonid native to tributaries of the Pacific Ocean in Asia and North America as well as much of the central, western, eastern, and especially the northern portions of the United States. The ocean going (anadromous) form (including those returning for spawning) are known as steelhead, or ocean trout (Australia). The species has been introduced for food or sport to at least 45 countries, and every continent except Antarctica. In some of these locations, such as Southern Europe, Australia and South America, they have had very serious negative impacts on upland native fish species, either by eating them, outcompeting them, transmitting contagious diseases, or hybridization with closely related species and subspecies that are native to western North America.

The species was originally named by Johann Julius Walbaum in 1792 based on type specimens from Kamchatka. Richardson named a specimen of this species Salmo gairdneri in 1836, and in 1855, W. P. Gibbons found a population and named it Salmo iridia, later corrected to Salmo irideus, however these names became deprecated once it was determined that Walbaum's type description was conspecific and therefore had precedence (see e.g. Behnke, 1966). More recently, DNA studies showed rainbow trout are genetically closer to Pacific salmon (Onchorhynchus species) than to brown trout (Salmo trutta) or Atlantic Salmon (Salmo salar), so the genus was changed.

Unlike the species' former name's epithet iridia (Latin: "rainbow"), the specific epithet mykiss derives from the local Kamchatkan name 'mykizha'; all of Walbaum's species names were based on Kamchatkan local names.

Physical characteristics

Rainbow trout are unusual in that there are two forms which sometimes share the same habitat. The anadromous form called "steelhead" migrates to the ocean, though they must return to fresh water to reproduce.

The freshwater form is called "rainbow trout", based on the broad red band along their sides. Steelhead are exactly the same species as rainbow trout. However, the difference is anadromy. After going to sea, their color changes, including loss of the red band. They stay at sea for 1-4 years, and return to fresh water to spawn. Rainbows stay in fresh water their whole lives.

It is a common misconception that "redband trout" and "rainbow trout" are the same fish. While closely related, redbands are a different subspecies, somewhat intermediate between rainbow and cutthroat trout (Oncorhynchus clarki). Redbands have larger spots and darker colors than rainbows and sometimes have cutthroat-like slash marks, but not the cutthroat's longer maxilla or hyoid (basibranchial) teeth. Redbands include anadromous steelhead forms such as the "redsides" of the Columbia River system above the Columbia River Gorge and similar fish from the Fraser River system above Fraser Canyon and the most inland parts of the Sacramento River, Klamath River and others flowing into the Pacific.