The United States Marine Corps (USMC), also referred to as the United States Marines or simply the Marines, is the maritime land force service branch of the United States Department of Defense. It is responsible for conducting expeditionary and amphibious operations through combined arms, implementing its own infantry, artillery, aerial, and special operations forces. The U.S. Marine Corps is one of the six armed forces of the United States and one of the eight uniformed services of the United States.

The Marine Corps has been part of the United States Department of the Navy since 30 June 1834 with its sister service, the United States Navy. The USMC operates installations on land and aboard sea-going amphibious warfare ships around the world. Additionally, several of the Marines' tactical aviation squadrons, primarily Marine Fighter Attack squadrons, are also embedded in Navy carrier air wings and operate from the aircraft carriers.

The history of the Marine Corps began when two battalions of Continental Marines were formed on 10 November 1775 in Philadelphia as a service branch of infantry troops capable of fighting both at sea and on shore. In the Pacific theater of World War II, the Corps took the lead in a massive campaign of amphibious warfare, advancing from island to island. As of December 2024, the USMC has around 169,000 active duty members and some 33,000 personnel in reserve.

As outlined in 10 U.S.C. § 5063 and as originally introduced under the National Security Act of 1947, the three primary areas of responsibility for the U.S. Marine Corps are:

This last clause derives from similar language in the congressional acts "For the Better Organization of the Marine Corps" of 1834 and "Establishing and Organizing a Marine Corps" of 1798. In 1951, the House of Representatives' Armed Services Committee called the clause "one of the most important statutory – and traditional – functions of the Marine Corps". It noted that the Corps has more often than not performed actions of a non-naval nature, including its famous actions in Tripoli, the War of 1812, Chapultepec, and numerous counterinsurgency and occupational duties (such as those in Central America, World War I, and the Korean War). While these actions are not accurately described as support of naval campaigns nor as amphibious warfare, their common thread is that they are of an expeditionary nature, using the mobility of the Navy to provide timely intervention in foreign affairs on behalf of American interests.

The Marine Band, dubbed the "President's Own" by John Adams, provides music for state functions at the White House. Marines from Ceremonial Companies A & B, quartered in Marine Barracks, Washington, D.C., guard presidential retreats, including Camp David, and the marines of the Executive Flight Detachment of HMX-1 provide helicopter transport to the president and vice president, with the radio call signs "Marine One" and "Marine Two", respectively. The Executive Flight Detachment also provides helicopter transport to Cabinet members and other VIPs. By authority of the 1946 Foreign Service Act, the Marine Security Guard of the Marine Embassy Security Command provide security for American embassies, legations, and consulates at more than 140 posts worldwide.

The relationship between the Department of State and the U.S. Marine Corps is nearly as old as the Corps itself. For over 200 years, Marines have served at the request of various secretaries of state. After World War II, an alert, disciplined force was needed to protect American embassies, consulates, and legations throughout the world. In 1947, a proposal was made that the Department of Defense furnish Marine Corps personnel for Foreign Service guard duty under the provisions of the Foreign Service Act of 1946. A formal Memorandum of Agreement was signed between the Department of State and the secretary of the Navy on 15 December 1948, and 83 Marines were deployed to overseas missions. During the first year of the program, 36 detachments were deployed worldwide.

The Marine Corps was founded to serve as an infantry unit aboard naval vessels and was responsible for the security of the ship and its crew by conducting offensive and defensive combat during boarding actions and defending the ship's officers from mutiny; to the latter end, their quarters on the ship were often strategically positioned between the officers' quarters and the rest of the vessel. Continental Marines manned raiding parties, both at sea and ashore. America's first amphibious assault landing occurred early in the Revolutionary War, on 3 March 1776, as the Marines gained control of Fort Montagu and Fort Nassau, a British ammunition depot and naval port in New Providence, the Bahamas. The role of the Marine Corps has expanded significantly since then; as the importance of its original naval mission declined with changing naval warfare doctrine and the professionalization of the naval service, the Corps adapted by focusing on formerly secondary missions ashore. The Advanced Base Doctrine of the early 20th century codified their combat duties ashore, outlining the use of Marines in the seizure of bases and other duties on land to support naval campaigns. In 1987, the USMC Sea School was closed; in 1998, all Marine Detachments on board ships were disbanded.