The Secretary of State for War, commonly called War Secretary, was a secretary of state in the Government of the United Kingdom, which existed from 1794 to 1801 and from 1854 to 1964. The Secretary of State for War headed the War Office and was assisted by a Parliamentary Under-Secretary of State for War, a Parliamentary Private Secretary who was also a Member of Parliament (MP), and a Military Secretary, who was a general.

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.