The Union navy is used to describe the United States Navy (USN) during the American Civil War, when it fought the Confederate States Navy (CSN). The term is sometimes used to describe vessels of war used on the rivers of the interior under the control of the Union army.

The primary missions of the Union navy were:

The system of naval bureaus was revised in the summer of 1862. Some of the older bureaus were rearranged or had their names altered. The most radical change was the creation of the Bureau of Steam Engineering.

To accomplish its wartime missions, the Union navy had to undergo a profound technical and institutional transformation. During the war, sailing vessels were completely supplanted by ships propelled by steam for combat purposes. Vessels of widely differing character were built from the keel up in response to peculiar problems they would encounter. Wooden hulls were first protected by armor plating and soon replaced by iron or steel throughout. Guns were reduced in number but increased in size and range; the reduction was partially compensated by mounting the guns in rotating turrets or pivoting the gun on a curved deck track so they could be turned to fire in any direction.

The institutional changes that were introduced during the war were equally significant. The Bureau of Steam Engineering was added to the bureau system, testimony to the U.S. Navy's conversion from sail to steam. Most important, from the standpoint of Army-Navy cooperation in joint operations, the set of officer ranks was redefined so that each rank in the U.S. Army had its equivalent in the U.S. Navy. Establishing the ranks of admirals also implied a change of naval doctrine, from one favoring single-ship operations to that of employing whole fleets.

A doctrinal shift took place at the same time. Before the war, the United States Navy emphasized single-ship operations, but the nature of the conflict soon made whole fleets necessary. Already at the Battle of Port Royal (November 7, 1861), 77 vessels, including 19 warships, were employed. This was the largest naval expedition ever sailed under the U.S. flag, but the record did not last long. Subsequent operations at New Orleans, Mobile, and several positions in the interior confirmed the importance of large fleets in modern naval operations.

At the start of the war, the Union navy had 42 ships in commission. Another 48 were laid up and listed as available for service as soon as crews could be assembled and trained, but few were appropriate. Most were sailing vessels, some were hopelessly outdated, and one (USS Michigan) served on Lake Erie and could not be moved to the ocean. During the war, the number in commission was increased by more than a factor 15, so that at the end the U.S. Navy had 671 vessels.

The variety of ship types represented was even more significant than the increase in raw numbers, some of the forms not previously seen in naval war. The nature of the conflict, much of which occurred in the continent's interior or shallow harbors along the coast, meant that vessels designed for use on the open seas were less useful than more specialized ships. To confront the forms of combat that came about, the federal government developed a new type of warship, the monitor, based on the original, USS Monitor. The U.S. Navy took over a class of armored river gunboats created for the U.S. Army but designed by naval personnel, the Eads gunboats. So-called double-enders were produced to maneuver in the confined waters of the rivers and harbors. The Union navy experimented with submarines before the Confederacy produced its famed CSS Hunley; the result, USS Alligator, failed primarily because of lack of suitable targets. Building on Confederate designs, the Union navy produced and used torpedo boats, small vessels that mounted spar torpedoes and were forerunners of both the modern torpedo and destroyer type of warship.