A ministry of defence or defense, also known as a department of defence or defense, is the part of a government responsible for matters of national 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.

A minister is a politician who heads a ministry, making and implementing decisions on policies in conjunction with the other ministers. In some jurisdictions the head of government is also a minister and is designated the 'prime minister', 'premier', 'chief minister', 'chancellor' or other title.