The American Defense Service Medal is a decoration of the United States military which was created in 1941 by Executive Order of President Franklin Roosevelt.

The purpose of the American Defense Service Medal is to recognize those members of the U.S. military who had served on active duty before America’s entry into the Second World War but during the initial years of the European conflict. The medal is therefore authorized to any military member who performed duty between September 8, 1939 and December 6, 1941. Members of the United States Army were required to perform at least one year of duty, during the above time period, while United States Navy, U.S. Coast Guard, and United States Marine Corps personnel were awarded the medal for any length of service during the eligable time frame.

To denote foreign and pre-war battle service, the American Defense Service Medal was authorized with the following devices:

• Foreign Service Clasp: Issued by the United States Army for military service outside the continental limits of the United States, including service in Alaska.
• Base Clasp: Issued by the U.S. Navy and United States Marine Corps for service outside the continental limits of the United States (service in either Alaska or Hawaii qualified).
• Fleet Clasp: Issued by the Navy, Marine Corps and United States Coast Guard for service on the high seas while regularly attached to any vessels of the Atlantic, Pacific, or Asiatic fleets as well as vessels of the Naval Transport Service and vessels operating directly under the Chief of Naval Operations.
• Sea Clasp: Issued by the Coast Guard for all other vessels and aircraft, not qualifying for the Fleet Clasp, which regularly conducted patrols at sea.
• "A" Device: Awarded to any member of the Navy who served duty in actual or potential belligerent contact with Axis Powers in the Atlantic Ocean between June 22 and December 7, 1941.
• Service star: Worn in lieu of clasps when wearing the American Defense Service Medal as a ribbon on a military uniform.

A similar decoration, known as the American Campaign Medal was created in 1942 for war time service within the borders of the United States of America.

See also: Awards and decorations of the United States military