The Battle of Powder River, also known as the Reynolds Battle, occurred on March 17, 1876, in Montana Territory, United States, as part of the Big Horn Expedition. The attack on a Northern Cheyenne and Oglala Lakota Indian encampment by Colonel Joseph J. Reynolds initiated the Great Sioux War of 1876. Although destroying a large amount of Indian property, the attack was poorly carried out and solidified Northern Cheyenne and Lakota Sioux resistance to the U.S. attempt to force them to sell the Black Hills and live on a reservation.

Joseph Jones Reynolds was an American engineer, educator, and military officer who fought in the American Civil War and the postbellum Indian Wars.