The United States Navy (USN) is the maritime service branch of the United States Department of Defense. It is the world's most powerful navy with the largest displacement, at 4.5 million tons in 2021. It has the world's largest aircraft carrier fleet, with eleven in service, one undergoing trials, two new carriers under construction, and six other carriers planned as of 2024. With 336,978 personnel on active duty and 101,583 in the Ready Reserve, the U.S. Navy is the third largest of the United States military service branches in terms of personnel. It has 299 deployable combat vessels and about 4,012 operational aircraft as of 18 July 2023. The U.S. Navy is one of six armed forces of the United States and one of eight uniformed services of the United States.

The United States Navy traces its origins to the Continental Navy, which was established during the American Revolutionary War and was effectively disbanded as a separate entity shortly thereafter. After suffering significant loss of goods and personnel at the hands of the Barbary pirates from Algiers, the United States Congress passed the Naval Act of 1794 for the construction of six heavy frigates, the first ships of the Navy. The United States Navy played a major role in the American Civil War by blockading the Confederacy and seizing control of its rivers. It played the central role in the World War II defeat of Imperial Japan. The United States Navy emerged from World War II as the most powerful navy in the world. The modern United States Navy maintains a sizable global presence, deploying in strength in such areas as the Western Pacific, the Mediterranean, and the Indian Ocean. It is a blue-water navy with the ability to project force onto the littoral regions of the world, engage in forward deployments during peacetime and rapidly respond to regional crises, making it a frequent actor in American foreign and military policy.

The United States Navy is part of the Department of the Navy, alongside the United States Marine Corps, which is its coequal sister service. The Department of the Navy is headed by the civilian secretary of the Navy. The Department of the Navy is itself a military department of the Department of Defense, which is headed by the secretary of defense. The chief of naval operations (CNO) is the most senior Navy officer serving in the Department of the Navy.

To recruit, train, equip, and organize to deliver combat ready Naval forces to win conflicts and wars while maintaining security and deterrence through sustained forward presence.

— Mission statement of the United States Navy.

The Navy's three primary areas of responsibility are:

U.S. Navy training manuals state that the overall mission of the armed forces is "to be prepared to conduct prompt and sustained combat operations in support of the national interest." The Navy's five enduring functions are: sea control, power projection, deterrence, maritime security, and sealift.

It follows then as certain as that night succeeds the day, that without a decisive naval force we can do nothing definitive, and with it, everything honorable and glorious.