The third USS Union was a heavy (1,114-ton) steamer with a powerful 12-inch rifled gun purchased by the United States Navy during the American Civil War.

Towards war's end, she was also assigned the role of dispatch boat and, because of her large size, of storeship, at the same time continuing to capture blockade runners.

She performed routine reconnaissance and dispatch duties on the Potomac River and, despite heavy fire from shore, burned a large Confederate schooner in Dumfries Creek on 11 October 1861.

Union suffered no casualties during the action and received special commendation for her daring exploit from the United States Secretary of the Navy, Gideon Welles.

She spent the remainder of the war operating between New York City; Hampton Roads; Port Royal, South Carolina; and points scattered along the Florida coast and the shore of the Gulf of Mexico.

On 14 January 1864, Union seized the steamer Mayflower and her cargo of cotton near Tampa Bay, Florida, and, on 26 April 1864, she captured the schooner O.K.

Union completed several dispatch and supply missions after the American Civil War ended and was decommissioned at New York City on 29 September 1865.