The War Office was a department of the British Government responsible for the administration of the British Army between 1857 and 1964, when its functions were transferred to the new Ministry of Defence (MoD). It was equivalent to the Admiralty, responsible for the Royal Navy (RN), and (much later) the Air Ministry, which oversaw the Royal Air Force (RAF). The name 'War Office' is also given to the former home of the department, located at the junction of Horse Guards Avenue and Whitehall in central London. The landmark building was sold on 1 March 2016 by HM Government for more than £350 million, on a 250 year lease for conversion into a luxury hotel and residential apartments.

Prior to 1855, 'War Office' signified the office of the Secretary at War. In the 17th and 18th centuries, a number of independent offices and individuals were responsible for various aspects of Army administration. The most important were the Commander-in-Chief of the Forces, the Secretary at War, and the twin Secretaries of State; most of whose military responsibilities were passed to a new Secretary of State for War in 1794. Others who performed specialist functions were the controller of army accounts, the Army Medical Board, the Commissariat Department, the Board of General Officers, the Judge Advocate General of the Armed Forces, the Commissary General of Muster, the Paymaster General of the forces, and (particularly with regard to the Militia) the Home Office.

The term War Department was initially used for the separate office of the Secretary of State for War; in 1855, the offices of Secretary at War and Secretary of State for War were amalgamated, and thereafter the terms War Office and War Department were used somewhat interchangeably.