The Vietnam Service Medal is a decoration of the United States military which was created in 1965 by order of President Lyndon B. Johnson. The medal is issued to recognize military service during the Vietnam War and is authorized to service members in every branch of the U.S. Armed Forces.

The Vietnam Service Medal is presented to any service member who served more than thirty consecutive days, or 60 non-consecutive days, in the Republic of Vietnam between the dates of November 15, 1961 and March 28, 1973. For those service members who supported Vietnam operations from another country (such as Thailand), the Vietnam Service Medal may be authorized if such activity was in direct support of Vietnam combat operations and if such combat support exceeded 30-60 days.

For the United States Navy, vessels operating in Vietnamese waters qualify for the Vietnam Service Medal provided that the naval vessel was engaged in direct support of Vietnam combat operations. The U.S. Air Force also grants the Vietnam Service Medal for 30-60 days of flight missions in Vietnamese air space, even if the home base of the flight mission was not within the Republic of Vietnam.

The Department of Defense established twenty nine military campaigns during the Vietnam War. For those service members participating in one or more campaigns, a bronze service star is worn on the Vietnam Service Medal. Silver service stars are issued in lieu of five bronze. Some campaigns apply to the military services while others are specific to a particular branch of the U.S. Armed Forces. The following are the established combat campaigns authorized for service stars to the Vietnam Service Medal.

Vietnam Initial Advisory Campaign (USAF): November 15 1961 to March 1, 1965
Vietnam Advisory Campaign (Army, Navy, USCG): March 15, 1962 to March 7, 1965
Vietnam Defense Campaign (Army, Navy USCG): March 8 to December 24, 1965
Vietnam Air Defensive Campaign (USAF): March 2, 1965 to January 30, 1966
Vietnam Counteroffensive (Army, Navy, USCG): December 25, 1965 to June 30, 1966
Vietnam Air Counteroffensive (USAF): January 31 to June 28, 1966
Vietnam Air Offensive (USAF): June 29, 1966 to March 8, 1967
Vietnam Counteroffensive, Phase II (Army, Navy, USCG): July 1, 1966 to May 31, 1967
Vietnam Air Offensive, Phase II (USAF): March 9, 1967 to March 31, 1968
Vietnam Counteroffensive, Phase III (Army, Navy, USCG): June 1, 1967 to January 29, 1968
Vietnam Air/Ground Campaign (USAF): January 22 to July 7, 1968
Tet Counteroffensive (Army, USAF, Navy, USCG): January 30 to April 1, 1968
Vietnam Air Offensive, Phase III (USAF): April 1 to October 31, 1968
Vietnam Counteroffensive, Phase IV (Army, Navy, USCG): April 2 to June 30, 1968
Vietnam Counteroffensive, Phase V (Army, Navy, USCG): July 1 to November 1, 1968
Vietnam Air Offensive, Phase IV (USAF): November 1, 1968 to February 22, 1969
Vietnam Counteroffensive, Phase VI (Army, Navy, USCG): Nov 2, 1968 to Feb 22, 1969
Tet 1969 Counteroffensive (Army, USAF, USCG): February 23 to June 8, 1969
Vietnam Summer-Fall 1969 (Army, Navy, USAF, USCG): June 9 to October 31, 1969
Vietnam Winter-Spring 1970 (Army, Navy, USAF, USCG): November 1, 1969 to April 30, 1970
Sanctuary Counteroffensive (Army, Navy, USAF, USCG): May 1 to June 30, 1970
Southwest Monsoon (USAF): July 1 to November 30, 1970
Vietnam Counteroffensive, Phase VII (Army, Navy, USCG): July 1, 1970 to June 30, 1971
Commando Hunt V (USAF): December 1, 1970 to May 14, 1971
Commando Hunt VI (USAF): May 15 to October 31, 1971
Consolidation I (Army, Navy, USCG): July 1 to November 30, 1971
Commando Hunt VII (USAF): November 1, 1971 to March 29, 1972
Consolidation II (Army, Navy, USCG): December 1, 1971 to March 29, 1972
Vietnam Cease Fire (Army, Navy, USAF, USCG): March 30, 1972 to March 28, 1973

The Vietnam Service Medal (VSM) is retroactive to 1961 and supercedes and replaces the Armed Forces Expeditionary Medal (AFEM) which was issued for Vietnam service prior to 1965. Defense Department regulations do not permit the simultaneous presentation of both the Vietnam Service Medal, and the Armed Forces Expeditionary Medal, for the same period of service in Vietnam, however the AFEM may be exchanged for the VSM upon request from a service member. Veterans of the Vietnam War may exchange the AFEM for the VSM and have military records updated to reflect the difference by contacting the National Personnel Records Center, which is the current agency that provides record corrections reflecting an AFEM upgrade to the Vietnam Service Medal.

The Republic of Vietnam also issued its own version of the Vietnam Service Medal, known as the Vietnam Campaign Medal. This is a separate decoration from the Vietnam Service Medal and is considered a foreign decoration by the U.S. military.