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 283 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. 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.