The United States Navy (USN) is the sea branch of the U.S. Armed Forces. It is one of the seven uniformed services of the United States. As of 31 December 2008, the U.S. Navy had about 331,682 personnel on active duty and 124,000 in the Navy Reserve. It operates 284 ships in active service and more than 3,700 aircraft. The U.S. Navy is the largest in the world; its battle fleet tonnage is greater than that of the next 13 largest combined. The U.S. Navy also has the world's largest carrier fleet, with 11 in service and one under construction.

The Navy traces its origins to the Continental Navy, which was established during the American Revolutionary War (1775–1783) and was essentially disbanded as a separate entity shortly thereafter. The United States Constitution provided the legal basis for a seaborne military force by giving Congress the power "to provide and maintain a navy".

Depredations against American shipping by Barbary Coast pirates in the Mediterranean Sea spurred Congress to employ this power by passing the Naval Act of 1794 ordering the construction and manning of six frigates. These ships were used to end most pirate activity off the Barbary Coast. In the twentieth century American blue-water navy capability was demonstrated by the 1907–1909 world tour of the Great White Fleet.

The 21st century United States Navy maintains a sizable global presence, deploying in such areas as East Asia, the Mediterranean, and the Middle East. It is a blue water navy with the ability to project force onto the littoral regions of the world, engage in forward areas during peacetime, and rapidly respond to regional crises, making it an active player in American foreign and defense policy.

The Navy is administratively managed by the Department of the Navy, which is headed by the civilian Secretary of the Navy. The Department of the Navy is itself a division of the Department of Defense, which is headed by the Secretary of Defense. The highest ranking Naval officer is the Chief of Naval Operations.

Mission
The mission of the Navy is to maintain, train and equip combat-ready Naval forces capable of winning wars, deterring aggression and maintaining freedom of the seas.
—Mission statement of the United States Navy
From the New Recruits Handbook:

The mission of the United States Navy is to protect and defend the right of the United States and our allies to move freely on the oceans and to protect our country against her enemies.
The United States Navy serves as a seaborne branch of the Military of the United States. 10 U.S.C. § 5062 prescribes the navy's three primary areas of responsibility:

"The preparation of naval forces necessary for the effective prosecution of war"
"The maintenance of naval aviation, including land-based naval aviation, air transport essential for naval operations and all air weapons and air techniques involved in the operations and activities of the Navy"
"The development of aircraft, weapons, tactics, technique, organization, and equipment of naval combat and service elements".
U.S. Navy training manuals state the mission of the U.S armed forces is "to prepare and conduct prompt and sustained combat operations in support of the national interest". As part of that establishment, the U.S. Navy's functions comprise sea control, power projection and nuclear deterrence, in addition to "sealift" duties.