Global Range: Pacific lampreys occur in rivers of the Pacific Coast from Alaska (apparently rare north of the Alaska Peninsula) to the Rio Santo Domingo in Baja California (Malibu Creek, Los Angeles County, seems to be the southernmost point of regular occurrence in California; scattered distribution south of San Luis Obispo County, with regular runs in the Santa Clara River) (Moyle 2002). Adults are wide-ranging in the Pacific Ocean from Japan to Baja California. Along the Pacific coast of Japan they have been found as far south as the Yuhutu River, Hokkaido, but the species is not known to spwan there (Scott and Crossman 1973, Lee et al. 1980, Page and Burr 1991, Wydoski and Whitney 2003). Landlocked parasitic populations are known from Goose Lake and its tributaries in Oregon and California; Upper Klamath Lake, Oregon (reported as upper Klamath River by Moyle 2002); Cottonwood Reservoir in Lake County, Oregon; Cowichan Lake and River, British Columbia (this population is now included in L. macrostoma); and apparently in Dworshak Reservoir on the North Fork of the Clearwater River in Idaho (not lamprey collected there since 1973) (Scott and Crossman 1973, Moyle et al. 1995, Wydoski and Whitney 2003). A recently (1963) landlocked population exists in Clair Engle Reservoir on the Trinity River, Trinity County, California (Moyle 2002).
In Washington, the species ranges long distances upstream in the Columbia, Snake, and Yakima river systems (Wydoski and Whitney 2003). Historically, the Pacific lamprey occurred as far upstream as Kettle Falls in the Columbia River and Spokane Falls on the Spokane River (Wydoski and Whitney 2003). Grand Coulee Dam, completed in 1941, and Chief Joseph Dam, completed in 1955, blocked upstream passage on the Columbia River. Today, the lamprey occurs upstream to the Hells Canyon Dam on the Snakes River (Wydoski and Whitney 2003). In California, Pacific lampreys are blocked by major barriers, such as Friant Dam on the San Joaquin River and Scott Dam on the Eel River (Moyle 2002).