H. L. Hunley, often referred to as Hunley, CSS H. L. Hunley, or as 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 the 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 successful attack, was lost along with her crew before she could return to base. The Confederacy lost 21 crewmen in 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.

On 17 February 1864, Hunley attacked and sank the 1,240-ton United States Navy screw sloop-of-war Housatonic, which had been on Union blockade-duty in Charleston's outer harbor. Hunley did not survive the attack and also sank, taking with her all eight members of her third crew, and was lost.

Finally located in 1995, Hunley was raised in 2000, and is on display in North Charleston, South Carolina, at the Warren Lasch Conservation Center on the Cooper River. Examination in 2012 of recovered Hunley artifacts suggests that the submarine was as close as 20 ft (6.1 m) to her target, Housatonic, when her deployed torpedo exploded, which caused the submarine's own loss.