Cutthroat trout were given the name Salmo clarki in honor of William Clark, who co-led the expedition of 1804–1806.

[12] In 1853, naturalist George Suckley while working for the Pacific Railroad Survey led by Isaac Stevens collected specimens of westslope cutthroat trout by fly fishing below the Great Falls on the Missouri River.

[13] In David Starr Jordan and Barton Warren Evermann's A Check-list of the Fishes and Fishlike Vertebrates of North and Middle America (1896), the name Salmo mykiss lewisi was given to Yellowstone trout or cut-throat trout and included a reference to specimens collected from the Missouri River by George Suckley.

[16] Thus, in 1989, taxonomic authorities moved the rainbow, cutthroat and other Pacific basin trout into the genus Oncorhynchus.

They can be distinguished from rainbow trout by the red, pink, or orange marking beneath the jaw (hence the name "cutthroat").

[13] Isolated populations of Westslope cutthroat trout exist in upper tributaries of the John Day River in the Strawberry Mountains of Oregon[30] and Columbia River tributaries along the eastern side of the Cascade Range in Washington.

[31] Genetically pure Westslope cutthroat trout have been extirpated throughout most of their historic range due to habitat loss and introduction of non-native species.

Reasons for the critical condition of the subspecies include habitat destruction from logging, road building, grazing, mining, urban development, agriculture and dams, introduction of non-native hatchery strains, competition and hybridization from introduced non-native fish species.