The Mississippi Sound is a sound along the Gulf Coast of the United States. It runs east-west along the southern coasts of Mississippi and Alabama, from the mouth of the Pearl River at the Mississippi-Louisiana state border to the Dauphin Island Bridge, a distance of about 90 miles (145 km). The sound is separated from the Gulf on its southern side by the Mississippi–Alabama barrier islands: Cat, Ship, Horn, West Petit Bois, Petit Bois, and Dauphin. Ship, Horn, West Petit Bois and Petit Bois Islands are part of the National Park Service's Gulf Islands National Seashore. Those islands separate the sound from the Gulf of Mexico. The sediment of the islands was created partly by the ancient Mississippi River when the St. Bernard Lobe of the Mississippi Delta was active over two thousand years ago. The expansion of the St. Bernard subdelta slowly isolated the Mississippi Sound from ocean dynamics of the open Gulf of Mexico.

In geography, a sound is a smaller body of water usually connected to a sea or an ocean. A sound may be an inlet that is deeper than a bight and wider than a fjord; or a narrow sea channel or an ocean channel between two land masses, such as a strait; or also a lagoon between a barrier island and the mainland.