The Lost River is a river in northern California and southwestern Oregon in the United States, approximately 70 mi (113 km) long. The river flows within an enclosed basin on the volcanic plateau southwest of the Klamath River watershed. Its waters provide irrigation in a farming and ranching region along the California-Oregon border.

It rises in two forks in northern Modoc County, California west of Goose Lake in the Modoc National Forest. The short North Fork and South Fork, each approximately 20 mi (32 km) long, flow west and join east of the Clear Lake Reservoir. The combined stream flows through the Clear Lake Reservoir, then flows in a northward loop through southern Klamath County, Oregon, re-entering California approximately 20 mi (32 km) southeast of Klamath Falls. It enters the north end of intermittent Tule Lake, approximately 7 mi (13 km) south of the Oregon-California border.