A ministry of defence or defense (see spelling differences), also known as a department of defence or defense, is the part of a government responsible for matters of defence and military forces, found in states where the government is divided into ministries or departments. Such a department usually includes all branches of the military, and is usually controlled by a defence minister or secretary of defense.

The role of a defence minister varies considerably from country to country; in some the minister is only in charge of general budget matters and procurement of equipment, while in others they are also an integral part of the operational military chain of command.

Historically, such departments were referred to as a ministry of war or department of war, although they generally had authority only over the army of a country, with a separate department governing other military branches. Prior to World War II, most "ministries of war" were army ministries, while the navy and the air force, if it existed as a separate branch, had their own departments.^[1] As late as 1953, for example, the Soviet Union had a "ministry of war" alongside a "ministry of the navy".

The tendency to consolidate and rename these departments, and to coordinate until then mostly separate components of defence (air, land, navy) arose after World War II.

• Department of Defence (1901–21) (Australia)
• Department of Defence (1921–39) (Australia)
• Department of Defence Co-ordination (Australia, 1939–1942)
• Department of Defence Production (Australia, 1951–1958)
• Department of Defence Support (Australia, 1982–1984)
• Ministry of Defence (Artsakh) (1995–2023)
• Ministry of Defence (Czechoslovakia) (1918–1992)
• Ministry of National Defence (East Germany) (1956–1990)
• Ministry of Defence (Republika Srpska) (1992–2005)
• Ministry of Defense (Soviet Union) (1953–1992)
• Ministry of Defence (1947–1964) (United Kingdom)

• List of current defence ministers

• Chief of defence
• Commander-in-chief
• War cabinet
• Secretariat of National Defense (Mexico)
• Ministry of War
• Defence diplomacy, refers to the pursuit of foreign policy objectives through the peaceful employment of defence resources and capabilities