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 the minister is 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. 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.