USS Onondaga was an ironclad monitor built for the Union Navy during the American Civil War. Commissioned in 1864, the ship spent her entire active career with the James River Flotilla covering the water approaches to the Confederate States capital of Richmond, Virginia, although her only notable engagement was the Battle of Trent's Reach. After the war, she was purchased by France where she served as a coastal defense ship in the French Navy (Marine Nationale).