The United States Department of War, also called the War Department (and occasionally War Office in the early years), was the United States Cabinet department originally responsible for the operation and maintenance of the United States Army, also bearing responsibility for naval affairs until the establishment of the Navy Department in 1798, and for most land-based air forces until the creation of the Department of the Air Force on September 18, 1947.

The Secretary of War, a civilian with such responsibilities as finance and purchases and a minor role in directing military affairs, headed the War Department throughout its existence.

The War Department existed from August 7, 1789 until September 18, 1947, when it split into the Department of the Army and the Department of the Air Force and joined the Department of the Navy as part of the new joint National Military Establishment (NME), renamed the United States Department of Defense in 1949.