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 Waveland, Mississippi to the Dauphin Island Bridge, a distance of about 145 kilometers (90 mi). The sound is bordered on its southern edge by the barrier islands - Cat, Ship, Horn, Petit Bois and Dauphin Islands - which are part of the National Park Service's Gulf Islands National Seashore. Those islands separate the sound from the Gulf of Mexico.
Large portions of the Mississippi Sound reach depths of about 6 meters (20 ft). Part of the Gulf Intracoastal Waterway traverses the sound with a project depth of 3.6 meters (12 ft). The waterway, maintained by the US Army Corps of Engineers, is designed for towboat and barge traffic. Most of its route through the sound is merely an imaginary line through water whose depth exceeds the project depth. A section west of Cat Island and the portion north of Dauphin Island rely on dredged channels marked by aids to navigation maintained by the US Coast Guard.
Deepwater ports along the sound include Gulfport and Pascagoula. Dredged ship channels running basically north-south connect those ports to the Gulf of Mexico, running between pairs of the barrier islands.
The Pascagoula River flows into the sound.