The first USS Casco was the first of a class of twenty 1,175-ton light-draft monitors built by Atlantic Works, Boston, Massachusetts for the Union Navy during the American Civil War.

John Ericsson developed the original design for the Casco-class monitors, but Chief Engineer Alban C. Stimers, the General Inspector of Ironclads, elaborated it.

The vessels received another redesign in the aftermath of Rear Admiral Samuel F. Du Pont's failed attack on Charleston, South Carolina, on 7 April 1863.

It was discovered that Stimers and his assistant Theodore Allen had failed to compensate properly for the weight his revisions added to the original plan and this resulted in excessive stress on the hull and a freeboard of only 3 inches (76 mm).

He was forced to raise the hulls of the monitors under construction by nearly two feet and the first few completed vessels had their turrets removed and a single pivot-mount 11 in (280 mm) Dahlgren cannon mounted.