Nominally anhydrous minerals (e.g., olivine, clinopyroxene, and orthopyroxene) in peridotite xenoliths collected from the Colorado Plateau and southern Basin and Range in western North America were systematically analyzed by Fourier transform infrared spectroscopy for water contents. Measured water contents range from 2 to 45 ppm for olivine, from 53 to 402 ppm for orthopyroxene, and from 171 to 957 ppm for clinopyroxene. The Colorado Plateau has the highest water contents (up to 45 ppm H2O in olivine, 402 ppm H2O in orthopyroxene, and 957 ppm H2O in clinopyroxene), while San Carlos in the southern Basin and Range has the lowest water contents (up to 4 ppm H2O in olivine, 82 ppm H2O in orthopyroxene, and 178 ppm H2O in clinopyroxene). With the exception of San Carlos, the olivine and pyroxenes from all other localities (Dish Hill, Grand Canyon, and Navajo) have water contents close to or higher than that inferred for the fertile asthenospheric mantle. We interpret the high water contents measured here to have been introduced into the base of the lithospheric mantle by rehydration associated with the subduction of the Farallon plate beneath North America during the early Cenozoic. Application of an updated flow law for dislocation creep of wet olivine to lithospheric mantle conditions beneath the Colorado Plateau predicts that for a given background shear stress, hydration alone can result in approximately 1 order of magnitude drop in the effective viscosity at the base of the lithosphere. If viscosity alone is used to distinguish the lithosphere from underlying asthenosphere, this suggests that hydration could have resulted in more than 10 km of lithospheric thinning. Viscosity reduction and lithospheric thinning of even larger extents (up to ∼100 km) are predicted when thicker lithosphere (such as Archean cratons) and larger water contents (up to water‐saturated conditions) are considered. If our interpretations are correct, the implications of our study go beyond western North America and hint at a possible way of recycling continental mantle, including cratonic mantle, back into the convecting mantle.