island in the Gulf of Mexico (Mexico, Gulf of), at the entrance to Mobile Bay off the southwest coast of Alabama, U.S., about 30 miles (50 km) south of Mobile. Included in Mobile county, the island is about 15 miles (25 km) long.

It was visited in 1699 by the explorer Pierre Le Moyne d'Iberville (Iberville, Pierre Le Moyne d'), who had been sent by Louis XIV to establish a base for French colonization of Louisiana. Originally called Massacre Island (because human bones were found on the shore), it was renamed (1707) for the title of the heir to the French throne. It briefly was a Spanish possession in 1762, when France ceded its territory east of the Mississippi River to Spain, but the next year it went to England by the Treaty of Paris; in 1780 it was reclaimed by Spain. Although the United States claimed it in 1803 as part of the Louisiana Purchase, possession was not gained until 1813. Fort Gaines, begun on the island's eastern point in 1821, was captured during the American Civil War by Union forces under Admiral David Farragut (Farragut, David) during the Battle of Mobile Bay (Mobile Bay, Battle of) (1864).

Dauphin Island now serves as a resort area. It is periodically buffeted by hurricanes (tropical cyclones) and can sustain overwash and wind damage during the storms. The fort is preserved as a historic site, and a bird sanctuary is located on the island. The Alabama Deep Sea Fishing Rodeo is held in July.