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.

Canonicus was transferred to the South Atlantic Blockading Squadron at Charleston, South Carolina, after the capture of Fort Fisher in January, and helped to capture one blockade runner. She was sent to Havana, Cuba, to search for the Confederate ironclad CSS Stonewall and became one of the first ironclads to visit a foreign port. The ship was intermittently in commission from 1872 until she was permanently decommissioned in 1877. Canonicus was exhibited at the Jamestown Exposition of 1907, before she was sold for scrap the following year.