A defence minister (or defense minister) is a cabinet position which regulates the armed forces in some sovereign nations. The minister usually has a very important role in a cabinet.

When cabinets first started appearing in the late 18th century, they were often called "Ministers of War" (U.S., "Secretary of War"), who were in charge of the land forces of a nation. Since the end of World War II, the change was made from war to defence, and also often involved putting the defence minister in charge of land, sea and air warfare. Another common reform which occurred at the end of World War II was to place the defence minister in a national security council or inner cabinet which allows the head of government to coordinate both military and diplomatic activities.

The Defence Ministry in some countries is a very important ministry, sometimes considered more important than the foreign minister. It is often assumed by the Prime Minister if war is common for that nation (for example, no less than five Prime Ministers of Israel have held the Defence (Security) Ministry during their Premiership). In many nations, it is the very strong convention that the defence minister be a civilian to highlight civilian control over the military. In less democratic countries, the minister is often a uniformed army official, and in several countries, he will also hold the position of the Armed Forces Commander.