Cutthroat trout are native to western North America. The cutthroat species has evolved through geographic isolation into many subspecies, each native to a different major drainage basin. Native cutthroat species are found along the Pacific Northwest coast, in the Cascade Range, the Great Basin, and throughout the Rocky Mountains. Some coastal populations are anadromous, living primarily in the Pacific Ocean as adults and returning to fresh water from fall through early spring to feed on insects and spawn. Most populations, however, stay in freshwater throughout their lives and are known as non-migratory, stream-resident or riverine populations. Anadromous fish may reach weights of 20 pounds (9 kg), but those fish which remain permanently in freshwater may only reach a weight of 2 pounds (1 kg). At least three subspecies are confined to isolated basins in the Great Basin and can tolerate saline or alkaline water.
Cutthroat trout vary widely in size, coloration, and habitats. Though their coloration can range from golden to gray to green on the back, depending on subspecies and habitat, all populations feature distinctive red, pink, or orange marks on the underside of the lower jaw; usually the easiest diagnostic of the species for the casual observer. As adults, different populations and subspecies of cutthroat can range from 6–40 inches (15–100 cm) in length making size an ineffective indicator as to species.
Cutthroat will readily interbreed with the closely related rainbow trout, producing fertile hybrids commonly called "cutbow". Cutthroat will also hybridize with the O. gilae subspecies, the Gila trout and Apache trout.