USS Essex was a 1000-ton ironclad river gunboat of the United States Army and later United States Navy during the American Civil War. It was named by her captain, William Porter, for his father's old sailing frigate, the USS Essex. This Essex was originally constructed in 1856 at New Albany, Indiana as a steam-powered ferry named New Era.

The Mississippi River Squadron was the Union brown-water naval squadron that operated on the western rivers during the American Civil War. It was initially created as a part of the Union Army, although it was commanded by naval officers, and was then known as the Western Gunboat Flotilla and sometimes as the Mississippi Flotilla. It received its final designation when it was transferred to the Union Navy at the beginning of October 1862.