USS Canonicus was a single-turret monitor built for the United States Navy during the American Civil War, the lead ship of her class. The ship spent most of her first year in service stationed up the James River, where she could support operations against Richmond and defend against a sortie by the Confederate ironclads of the James River Squadron. She engaged Confederate artillery batteries during the year and later participated in both attacks on Fort Fisher, defending the approaches to Wilmington, North Carolina, from December 1864 to January 1865.