H. L. Hunley, also known as the Hunley, CSS H. L. Hunley, or CSS Hunley, was a submarine of the Confederate States of America that played a small part in the American Civil War. Hunley demonstrated the advantages and dangers of undersea warfare. She was the first combat submarine to sink a warship (USS Housatonic), although Hunley was not completely submerged and, following her attack, was lost along with her crew before she could return to base. Twenty-one crewmen died in the three sinkings of Hunley during her short career. She was named for her inventor, Horace Lawson Hunley, shortly after she was taken into government service under the control of the Confederate States Army at Charleston, South Carolina.

Hunley, nearly 40 ft (12 m) long, was built at Mobile, Alabama, and launched in July 1863. She was then shipped by rail on 12 August 1863 to Charleston. Hunley (then referred to as the "fish boat", the "fish torpedo boat", or the "porpoise") sank on 29 August 1863 during a test run, killing five members of her crew. She sank again on 15 October 1863, killing all eight of her second crew, including Horace Lawson Hunley himself, who was aboard at the time, even though he was not a member of the Confederate military. Both times Hunley was raised and returned to service.