The scientific name of the rainbow trout is Oncorhynchus mykiss.^[3] The species was originally named by German naturalist andtaxonomist Johann Julius Walbaum in 1792 based on type specimens from the Kamchatka Peninsula in Siberia. Walbaum's original species name, mykiss, was derived from the local Kamchatkan name used for the fish, mykizha. The name of the genus is from theGreek onkos ("hook") and rynchos ("nose"), in reference to the hooked jaws of males in the mating season (the "kype").^[4]

Sir John Richardson, a Scottish naturalist, named a specimen of this species Salmo gairdneri in 1836 to honor Meredith Gairdner, aHudson's Bay Company surgeon at Fort Vancouver on the Columbia River who provided Richardson with specimens.^[5] In 1855, William P. Gibbons, the curator of Geology and Mineralogy^[6] at the California Academy of Sciences, found a population and named itSalmo 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.^[7] In 1989, morphological and genetic studies indicated that trout of thePacific basin were genetically closer to Pacific salmon (Oncorhynchus species) than to the Salmos – brown trout (Salmo trutta) orAtlantic salmon (Salmo salar) of the Atlantic basin.^[8] Thus, in 1989, taxonomic authorities moved the rainbow, cutthroat and other Pacific basin trout into the genus Oncorhynchus.^[4] Walbaum's name had precedence, so the species name Oncorhynchus mykissbecame 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.^[4] Anadromous forms of the coastal rainbow trout (O. m. irideus)or redband trout (O. m. gairdneri) are commonly known as steelhead.^[3]