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.
3/9 was reactivated under the 2nd Marine Division in May 2008 as the Marines expanded as a result of the continuing War on Terror.
