The Ashnola River is a tributary of the Similkameen River, rising in the northeastern part of the North Cascades in Washington, United States, and flowing north into British Columbia, Canada to join the Similkameen River about halfway along that river's course between the towns of Princeton and Keremeos. The river crosses the international boundary at (49°00′00″N120°19′37″W) and transits Cathedral Provincial Park.