USS Canonicus was a single-turret monitor built for the United States Navy during the American Civil War, the lead ship of her class.

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.

[5] The exposed sides of the hull were protected by five layers of 1-inch (25 mm) wrought iron plates, backed by wood.

A "rifle screen" of 1⁄2-inch (13 mm) armor 3 feet (0.9 m) high was installed on the top of the turret to protect the crew against Confederate snipers based on a suggestion by Commander Tunis A. M. Craven, captain of her sister ship Tecumseh.

[7] The ship's construction was delayed by multiple changes ordered while she was being built that reflected battle experience with earlier monitors.

This included the rebuilding of the turrets and pilot houses to increase their armor thickness from 8 inches (203 mm) to 10 inches, and to replace the bolts that secured their armor plates together with rivets, to prevent them from being knocked loose by the shock of impact from shells striking the turret.

[1] Canonicus sailed from Boston, on 22 April 1864, and arrived at Newport News, Virginia, on 3 May, for service with the James River Flotilla.

On 21 June, Commander Craven, of Tecumseh, spotted a line of breastworks that the Confederates were building at Howlett's Farm, and his ship opened fire at the workers.

One shell pierced the upper part of the funnel and the other struck the deck and ricocheted into the turret; no one was wounded or killed during the engagement.

In the first engagement on 24–25 December, Canonicus anchored at ranges from 900–1,200 yd (820–1,100 m) and fired 144 rounds, Lieutenant Commander George Belknap claiming to have dismounted two Confederate guns.

The following month, Canonicus and the monitors Monadnock and Catskill captured a blockade runner that had run aground on Sullivan's Island, on the night of 18 February.