Sea Islands, chain of more than 100 low islands off the Atlantic coast of S.C., Ga., and N Fla., extending from the Santee River to the St. Johns River. The ocean side of the islands is generally sandy; the side facing the mainland is marshy. The islands have a humid, subtropical climate, with hot summers, warm winters, and rain throughout the year. Once the center of the Gullah culture of former slaves, most of the islands have succumbed to modernization, and much of the African-American population has moved away. Some islands remain uninhabited; others are resorts and wildlife sanctuaries. The Intracoastal Waterway passes through the Sea Islands.

Morris Island, Fort Sumter, and other islands lie in and around Charleston harbor. Beaufort (1990 pop. 9,576), on Port Royal Island, is the main city of the Sea Islands. Parris Island is the Atlantic coast recruit-training center for the U.S. marine corps. St. Simons Island, Sea Island, and Jekyll Island (also called the Golden Isles), near Brunswick, Ga., are popular resorts. St. Simons is joined to the mainland at Brunswick by a causeway. Jekyll Island, once the site of a club for Northern millionaires, is now a state park. Cumberland Island, largest of the Sea Islands, c.22 mi (35 km) long and from 1 to 5 mi (1.6–8 km) wide, has been designated a national seashore (see National Parks and Monuments, table). Other notable islands are the Isle of Palms, Johns, Edisto, and Hilton Head, which is a major resort.

The Spanish were the first Europeans to explore and inhabit the islands, setting up missions and garrisons in the 16th cent.; a French attmept at settlement in 1560s was thwarted by the Spanish. Spain's outposts were later abandoned as the English steadily advanced in the area. James Oglethorpe, founder of the Georgia colony, built Fort Frederica on St. Simons Island between 1736 and 1754, during the English-Spanish struggle for control of the SE United States. The ruins of the fort are a national monument.

The Sea Islands were the first important cotton-growing area in North America. In the early 19th cent., St. Helena and Port Royal Island became the seats of large plantations that grew long-staple, Sea-Island cotton. The Union invasion in the Civil War and the distribution of land by the federal government to newly freed slaves after the war affected the wealth of the planters. With the coming of the boll weevil (c.1920), cotton culture gave way to diversified farming, including the growing of corn, potatoes, and peanuts. Phosphate mining, oystering, shrimping, and fishing also became important, and tourism and local military installations are now significant contributors to the local economy.