The Global War on Terrorism Service Medal is a military decoration of the United States military which was created by Presidential Order of George W. Bush in March 2003. The decoration recognizes those military service members who have performed service in the War on Terrorism from September 11, 2001 to a date to be determined.

To be awarded the Global War on Terrorism Service Medal, a military service member must perform duty in a designated anti-terrorism operation for a period of either 30 consecutive or 60 non-consecutive days of duty. For those who were engaged in combat, killed, or wounded in the line of the duty the time requirement is waived.

The medal is also awarded to personnel who perform support duty of an anti-terrorism operation but do not directly participate. The phrase "support" is defined as any administrative, logistics, planning, operational, technical, or readiness activity which provides support to an operation of the Global War on Terrorism. As of 2005, this blanket term has effectively awarded the Global War on Terrorism Service Medal to most personnel of the United States armed forces who performed service after September of 2001.

The initial authorized operation for the Global War on Terrorism Service Medal was the so called "Airport Security Operation" which occured between September 27, 2001 and May 31, 2002. Additional operations, for which the Global War on Terrorism Service Medal is authorized, include the active military campaigns of Operation Enduring Freedom, Operation Noble Eagle, and Operation Iraqi Freedom. Future operations are at the discretion of United States component commanders upon approval from the United States Department of Defense.

A similar decoration, known as the Global War on Terrorism Expeditionary Medal, also was created under the same Presidential Order that authorized the GWOT Service Medal. The primary difference between the two awards is that the service medal is intended for those who performed duty within the United States while the expeditionary medal recognizes those who were deployed to foreign countries.

For those participating in multiple operations, both the GWOT Service and GWOT Expeditionary Medal may be authorized, however both medals cannot be bestowed for service in a single operation. In addition, the Global War on Terrorism Expeditionary Medal may not be awarded, in lieu of the service medal, if duty was performed within the United States and not in an overseas deployed status.