The Fortymile River is a 60-mile (97 km) tributary of the Yukon River in the U.S. state of Alaska and the Canadian territory of Yukon. Beginning at the confluence of its north and south forks in the Southeast Fairbanks Census Area, the Fortymile flows generally northeast into Canada to meet the larger river 32 miles (51 km) southeast of Eagle, Alaska.

The Yukon River is a major watercourse of northwestern North America. From its source in British Columbia, it flows through Canada's territory of Yukon. The lower half of the river continues westward through the U.S. state of Alaska. The river is 3,190 kilometres (1,980 mi) long and empties into the Bering Sea at the Yukon–Kuskokwim Delta. The average flow is 6,400–7,000 m3/s (230,000–250,000 cu ft/s). The total drainage area is 833,000 km2 (321,500 sq mi), of which 323,800 km2 (125,000 sq mi) lies in Canada. The total area is more than 25% larger than Texas or Alberta.