The Union Navy was the United States Navy (USN) during the American Civil War, when it fought the Confederate States Navy (CSN). The term is sometimes used carelessly to include vessels of war used on the rivers of the interior while they were under the control of the United States Army, also called the Union Army.

The Confederate States Navy (CSN) was the naval branch of the Confederate States Armed Forces, established by an act of the Confederate States Congress on February 21, 1861. It was responsible for Confederate naval operations during the American Civil War against the United States's Union Navy.