group of barrier islands along the coast of the Gulf of Mexico (Mexico, Gulf of) in the southern United States, located near Gulfport and Biloxi, southern Mississippi, and near Pensacola, northwestern Florida. It also includes a mainland portion, and some four-fifths of the national seashore is underwater. The national seashore stretches some 160 miles (260 km) from Cat Island in Mississippi (west) to Santa Rosa Island in Florida (east) and is divided by the state of Alabama. The total area of 215 square miles (556 square km) is nearly equally apportioned between Florida and Mississippi. The national seashore was created in 1971.

Offshore in Florida, Perdido Key and Santa Rosa Island (which includes Fort Pickens, completed 1834) have historical ruins and four areas of white sandy beaches. The Naval Live Oaks area and Fort Barrancas are located on the Florida mainland. In Mississippi the offshore islands West Ship, East Ship, Horn, and Petit Bois have historical ruins also, notably Fort Massachusetts (completed in 1866) on West Ship Island, and wilderness areas accessible only by boat. At Ocean Springs, on the mainland near Biloxi, is the headquarters of the Mississippi district and the Davis Bayou area, which includes a nature trail, picnic areas, and a campground. The main headquarters of the national seashore are in Gulf Breeze, Florida.

The islands are of white quartz sand and are constantly being carved by weather and waves. Salt marshes, dunes covered with sea oats, and maritime forests of live oaks and pines are part of the national seashore. Animal life includes sea turtles, alligators, a wide variety of birds (such as plovers, terns, sanderlings, and pelicans), and abundant fish, shellfish, and other sea life.