The Secretary at War was a political position in the English and later British government, with some responsibility over the administration and organization of the Army, but not over military policy. The Secretary at War ran the War Office. After 1794 it was occasionally a Cabinet-level position, although it was considered of subordinate rank to the Secretaries of State. The position was combined with that of Secretary of State for War in 1854 and abolished in 1863.

The War Office has referred to several British government organisations in history, all relating to the army. It 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 at that time, equivalent to the Admiralty, responsible for the Royal Navy (RN), and 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.