Thief Valley Reservoir is a large reservoir on the Powder River in Eastern Oregon, United States. Primarily used for irrigation purposes, it lies at an elevation of approximately , covers an area of , and impounds . It provides good angling opportunities for rainbow trout in years of high water.
The reservoir was created in 1932 by the Thief Valley Dam, a project of the United States Bureau of Reclamation. The dam is a concrete-slab-and-buttress Ambursen structure, high and long. The dam was designed by Frank A. Banks.