Marine expeditionary units (MEU, pronounced as one syllable "M'you" IPA: ) are the smallest air-ground task forces (MAGTF) in the United States Fleet Marine Force. Each MEU is an expeditionary quick reaction force, deployed and ready for immediate response to any crisis, whether it be natural disaster or combat missions. Marine amphibious unit (MAU) was the name used until the late 1980s.

An MEU is normally composed of: a reinforced USMC infantry battalion (designated as a Battalion Landing Team) as the ground combat element, a composite medium tiltrotor squadron forming the aviation combat element, a combat logistics battalion providing the logistics combat element, and a company-size command element serving as the MEU headquarters group. Troop strength is about 2,200(Normal and Peacetime)-4,400(Mobilization and Wartime) members and usually commanded by a colonel, and is deployed from amphibious assault ships. Currently, an MEU embarks personnel and equipment onto the amphibious warfare ships of an expeditionary strike group (ESG) which also includes escort warships and submarines to protect them from air, surface, and submarine threats. For further protection and strong air support, an ESG is often deployed along with one or more carrier strike groups.