The U.S. Military Assistance Command, Vietnam (MACV) was a joint-service command of the United States Department of Defense, composed of forces from the United States Army, United States Navy, and United States Air Force, as well as their respective special operations forces.

MACV was created on 8 February 1962, in response to the increase in United States military assistance to South Vietnam. MACV was implemented to assist and oversee the Military Assistance Advisory Group (MAAG) Vietnam while the Viet Cong insurgency was under way. It was reorganized on 15 May 1964 and absorbed MAAG Vietnam when the deployment of combat units became too large for the advisory group to control.

General Paul D. Harkins was the first commanding general of MACV (COMUSMACV), and was previously the commander of MAAG Vietnam. After reorganization he was succeeded by General William Westmoreland in June 1964, followed by General Creighton W. Abrams (July 1968) and General Frederick C. Weyand (June 1972).

MACV was disestablished on 29 March 1973 and replaced by the Defense Attaché Office (DAO), Saigon. The DAO performed many of the same roles of MACV within the restrictions imposed by the Paris Peace Accords until the Fall of Saigon.

Admiral Harry D. Felt, Commander-in-Chief, Pacific, established the U.S. Military Assistance Command, Vietnam, on 8 February 1962, as a subordinate unified command under his control. Lieutenant General Paul D. Harkins, the Deputy Commander in Chief, U.S. Army, Pacific, who, as the commander-designate for the task force headquarters (HQ) in the event of operations in Southeast Asia, had participated in the planning for such operations, was appointed commander and promoted to general.

Harkins became the senior U.S. military commander in South Vietnam and responsible for U.S. military policy, operations and assistance there. Harkins had the task of advising the South Vietnamese government on security, organization, and employment of their military and paramilitary forces. As provided for in the organization of the task force headquarters in the contingency plans, MACV's commander was also his own Army component commander. With an initial authorized strength of 216 men (113 Army), MACV was envisaged as a temporary HQ that would be withdrawn once the Viet Cong insurgency was brought under control. In that event, the Military Assistance Advisory Group would be restored to its former position as the principal U.S. headquarters in South Vietnam. For this reason, the MAAG was retained as a separate headquarters.

In March 1962 Headquarters, United States Army Pacific, removed the "provisional" designation from the U.S. Army Support Group, Vietnam, attached it to United States Army Ryukyu Islands for administrative and logistical support, and made its commanding officer the deputy Army component commander under MACV. All U.S. Army units in South Vietnam, excluding advisory attachments, were assigned to the Army Support Group for administrative and logistical needs. Over the course of 1962 U.S. military strength in South Vietnam rose from about 1,000 to over 11,000 personnel. Each service continued to provide its own logistical support.

Throughout 1963 the duties of the U.S. Army Support Group steadily increased, particularly regarding to combat support activities and logistics. During the year, the U.S. buildup continued, especially in aviation, communications, intelligence, special warfare and logistic units, reaching a total of 17,068 men, of which 10,916 were Army. Because of this expansion, the commanding general, General Joseph Warren Stilwell Jr. late in 1963 proposed that the name of the support group be changed to U.S. Army Support Command, Vietnam. Harkins concurred and General James Francis Collins, commander of United States Army, Pacific and Admiral Felt approved the redesignation. The new designation went into effect on 1 March 1964.