This species can be found only in the United States and Canada, where it occurs mostly west of the Rocky Mountains.

This species of pearl mussel is found in Pacific drainages from California north to British Columbia and southern Alaska.

[3][4] However, this species is still common throughout parts of the northern Rocky Mountains,[2] although some populations in Montana may be declining.

[5] The most likely explanation for this distribution is headwater capture, where pre-glacial watersheds were cut and reconfigured by glacial advance or retreat.

Westslope cutthroat trout are thought to have crossed the Continental Divide from the West into the headwaters of the present-day Missouri River during the Pleistocene glaciation, more that 20,000 years ago.