A Ministry of Defence or Defense (see spelling differences), also known as a Department of Defence or Defense, is the common name for a part of the government found in states where the government is divided into ministries or departments, responsible for matters of defence. Such a department usually includes all branches of the military, and is usually controlled by a Defence Minister, Minister of Defence, or Secretary of Defense.

Historically, such departments were referred to as a Ministry of War or Department of War, although such departments generally had authority only over the army of a country, with a separate department governing other military branches. Prior to WWII, 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 Soviets for instance had a "Ministry of War" alongside a "Ministry of the Navy".

The tendency to consolidate and rename these departments to highlight their purpose as providing "defence" arose after World War II. In India, for example, "the name of the War Department was found objectionable in the sense that the concept had an aggressive aspect which was inconsistent with Indian traditions".