USS Iosco, a 1173-ton Sassacus class "double-ender" steam gunboat, was built at Bath, Maine. Commissioned in April 1864, she protected shipping in the Gulf of Saint Lawrence during the summer and was then sent to join the North Atlantic Blockading Squadron. While operating off Wilmington, North Carolina, on 21 November, Iosco captured the blockade runner Sybil.

On Christmas Eve 1864, Iosco participated in the first amphibious attack on Fort Fisher, N.C. which protected Wilmington, one of the South's most active centers of blockade running and her last port of entry for European aid. Her guns engaged the batteries at Mound Fort and succeeded in shooting down the Confederate flag which flew above the works. During the firing a Confederate shot carried away the head of Iosco's foremast. The next day, she led nine other ships in an attack on the fortress, closing the shore as near as her draft would permit. Meanwhile her boats dragged the channel for torpedoes. Throughout the operation she protected the right flank of the Union troops ashore until they reembarked under orders from the Army commander, Major General B. F. Butler 27 December 1864.

A fortnight later Iosco was again in the thick of the fighting during the second attack on Fort Fisher. She assisted the landing of troops and covered the right flank of the Army as it fought on shore 13 January 1865. Forty-four of her own men fought beside the soldiers on the beaches while her cannon fired at the mound until the Confederates surrendered 15 January. The remainder of Iosco's wartime service was in the North Carolina Sounds carrying out operations as Confederate resistance ceased, taking part in an expedition up the Roanoke River in mid-May. She sailed north 15 July and decommissioned 28 July 1865. Her engines were removed and her hull turned over to the Bureau of Construction and Repair for service as a coal hulk at the New York Navy Yard in February 1868.

USS Iosco