USS Merrimack, also improperly Merrimac, was a steam frigate, best known as the hull upon which the ironclad warship CSS Virginia was constructed during the American Civil War.

Merrimack was launched by the Boston Navy Yard 15 June 1855, sponsored by Mary E. Simmons, and commissioned 20 February 1856, Captain Garrett J. Pendergrast in command.

The day before the firing on Fort Sumter, Welles directed that "great vigilance be exercised in guarding and protecting" Norfolk Navy Yard and her ships.

[3] The Confederacy, in desperate need of ships, raised Merrimack and rebuilt her as an ironclad ram, according to a design prepared by Lt. John Mercer Brooke, CSN.

Commissioned as CSS Virginia 17 February 1862, the ironclad was the hope of the Confederacy to destroy the wooden ships in Hampton Roads, and to end the Union blockade which had already seriously impeded the Confederate war effort.