The 3rd Battalion, 9th Marines (3/9) is an infantry battalion of the United States Marine Corps. Formed during World War I it served until the early 1990s when it was redesignated as 3rd Battalion, 4th Marines (3/4) during a realignment and renumbering of the Marine Corps' infantry battalions, following the deactivation of the 9th Marine Regiment. The 3rd Battalion, 9th Marines was initially a subordinate unit of the 9th Marine Regiment, 3rd Marine Division, but was later operationally transferred to the 1st Marine Division as a subordinate unit of the 7th Marine Regiment where it remained until its redesignation as 3/4.

The 3rd Marine Division is a division of the United States Marine Corps based at Camp Courtney, Marine Corps Base Camp Smedley D. Butler in Okinawa, Japan. It is one of three active duty infantry divisions in the Marine Corps and together with the 1st Marine Aircraft Wing (1stMAW) and the 3rd Marine Logistics Group forms the III Marine Expeditionary Force. The division was first formed during World War II and saw four years of continuous combat in the Vietnam War. Today, elements of the 3rd Marine Division are continuously forward deployed and forward postured to carry out the US Government's mission of a Free and Open Indo-Pacific in conjunction with its sister services.