Fort Sumter is a historical sea fort located near Charleston, South Carolina. Constructed on an artificial island at the entrance of Charleston Harbor in 1829, the fort was built in response to the War of 1812, which had exposed the inadequacy of existing American coastal fortifications to defend against naval attacks. Fort Sumter was still incomplete in 1861 when it was attacked by Confederate Forces during the Battle of Fort Sumter on April 12, sparking the American Civil War; the fort was severely damaged during the battle and left in ruins. Although there were some efforts at reconstruction after the war, Fort Sumter as conceived was never completed. Since the middle of the 20th century, the fort has been open to the public as part of the Fort Sumter and Fort Moultrie National Historical Park, operated by the National Park Service.

Named after Continental Army officer Thomas Sumter, Fort Sumter was ordered in response to the War of 1812, which had exposed the inadequacy of existing American coastal fortifications to defend against naval attacks. It was built near Charleston, South Carolina, as part of the third system of U.S. fortifications to protect American harbors from a naval invasion. Constructed on an artificial island in the middle of the channel that provides Charleston with natural shelter, Fort Sumter was intended to dominate the harbor, reinforcing the protection provided by the shore artillery batteries at Fort Moultrie, Fort Wagner, and Fort Gregg.

The artificial island was originally a sand bar. In 1827, a group of engineers carried out depth sounding and concluded that it was a suitable location for a fort. Construction began in 1829. Seventy thousand tons of granite were transported from New England to build up the artificial island. By 1834, a timber foundation that was several feet beneath the water had been laid. However, the decision was made to build a (stronger) brick fort.

The brick fort is five-sided, 170 to 190 feet (52 to 58 m) long, with walls five feet (1.5 m) thick, standing 50 feet (15.2 m) over the low tide mark. Although never completed, it was designed to house 650 men and 135 guns in three tiers of gun emplacements.

Construction dragged out because of title problems, then problems with funding such a large and technically challenging project. Unpleasant weather and disease made the situation worse. The exterior was eventually finished, but the interior and armaments were never completed.

Early in the 19th century, South Carolina had owned multiple forts, namely Fort Moultrie, Castle Pinckney, and Fort Johnson, but ceded them, along with sites for the future erection of forts, to the United States in 1805. The forts were of questionable military value and costly to maintain, so when asked to cede them, the state complied. This was not the last time that South Carolina would cede forts to the United States; on December 17, 1836, South Carolina officially ceded all "right, title and, claim" to the site of Fort Sumter to the United States.

Fort Sumter is notable for two battles, the first of which began the American Civil War. It was one of a number of special forts planned after the War of 1812, combining high walls and heavy masonry, and classified as Third System, as a grade of structural integrity. Work started in 1829, but was incomplete by 1861, when the Civil War began.

The attack on Fort Sumter is generally taken as the beginning of the American Civil War—the first shots fired. Certainly it was so taken at the time—citizens of Charleston were celebrating. The First Battle of Fort Sumter began on April 12, 1861, when South Carolina Militia artillery fired from shore on the Union garrison. These were (both sides agreed) the first shots of the war. The bombardment continued all day, watched by many happy civilians. The fort had been cut off from its supply line and surrendered the next day. Major Robert Anderson took the flag with him as they evacuated.