The I Marine Amphibious Corps, or I MAC, was a formation of the United States Marine Corps.
It was created on 1 October 1942, with most of the staff transferred from Amphibious Corps, Pacific Fleet (ACPF). It was then deployed to the South Pacific Area – a U.S.-led multinational military command active during World War II that was a part of the U.S. Pacific Ocean Areas – first to Hawaii, then to New Caledonia.
When the Marine Corps was not satisfied with the leadership of its commander, Major General Clayton B. Vogel, Commandant Thomas Holcomb ordered Alexander Vandegrift to take command. Vandegrift, the commander of the 1st Marine Division during the Battle of Guadalcanal was promoted to lieutenant general.
General Vandegrift carried on in command of the Corps when its next-designated commander for the invasion of Bougainville, Major General Charles D. Barrett, died following a fall from the balcony of his quarters in New Caledonia. General Vandegrift led I MAC during the invasion and was later made the Commandant.
Consisting of the 3rd Marine Division, the 37th Infantry Division of the United States Army, and 8th New Zealand Brigade of the 3rd New Zealand Division, it conducted the invasion of Bougainville.
On 15 April 1944, I MAC was renamed the III Amphibious Corps.