Sir John Richardson, a Scottish naturalist, named a specimen of this species Salmo gairdneri in 1836 to honor Meredith Gairdner, a Hudson's Bay Company surgeon at Fort Vancouver on the Columbia River who provided Richardson with specimens. In 1855, William P. Gibbons, the curator of Geology and Mineralogy at the California Academy of Sciences, found a population and named it Salmo iridia (Latin: rainbow), later corrected to Salmo irideus. These names faded once it was determined that Walbaum's description of type specimens was conspecific and therefore had precedence. In 1989, morphological and genetic studies indicated that trout of the Pacific basin were genetically closer to Pacific salmon (Oncorhynchus species) than to the Salmos – brown trout (Salmo trutta) or Atlantic salmon (Salmo salar) of the Atlantic basin. Thus, in 1989, taxonomic authorities moved the rainbow, cutthroat and other Pacific basin trout into the genus Oncorhynchus. Walbaum's name had precedence, so the species name Oncorhynchus mykiss became the scientific name of the rainbow trout. The previous species names irideus and gairdneri were adopted as subspecies names for the coastal rainbow and Columbia River redband trout, respectively. Anadromous forms of the coastal rainbow trout (O. m. irideus) or redband trout (O. m. gairdneri) are commonly known as steelhead.
Adult freshwater stream rainbow trout average between 1 and 5 lb (0.5 and 2.3 kg), while lake-dwelling and anadromous forms may reach 20 lb (9 kg). Coloration varies widely based on subspecies , forms and habitat . Adult fish are distinguished by a broad reddish stripe along the lateral line , from gills to the tail, which is most vivid in breeding males.