was a Canonicus-class monitor built for the Union Navy during the American Civil War. The vessel was assigned to the James River Flotilla of the North Atlantic Blockading Squadron upon completion in September 1864. The ship spent most of her time stationed up the James River where she could support operations against Richmond and defend against sorties by the Confederate ironclads of the James River Squadron. She engaged Confederate artillery batteries during the year and later participated in both the first and second battles of Fort Fisher, defending the approaches to Wilmington, North Carolina, in December 1864 – January 1865. Mahopac returned to the James River after the capture of Fort Fisher and remained there until Richmond, Virginia was occupied in early April.