arm of the Gulf of Mexico (Mexico, Gulf of), extending southeastward along the southern coast of Louisiana, U.S., for 21 miles (34 km) from Point Chevreuil to Point Au Fer on Point Au Fer Island. The bay is 10 miles (16 km) wide, and Four League Bay extends another 11 miles (18 km) to the southeast. Eugene Island lies on a long shell reef extending 25 miles (40 km) northwest of Point Au Fer. The Lower Atchafalaya River (Atchafalaya River) links the bay with Morgan City, 15 miles (24 km) north, where the Gulf Intracoastal Waterway meets the Plaquemine–Morgan City Waterway and connects to the Mississippi River system.

The bay's reefs contain oyster shells, which provide road ballast and raw materials for making lime and cement at many coastal factories. There are many natural gas and oil fields in the area. The bay also is the focus of the Atchafalaya Delta Wildlife Management Area, which occupies some 214 square miles (554 square km) of water and land.