The Atlantic Coastal Plain is the rather flat stretch of land that borders the Atlantic Ocean (including the Gulf of Mexico). It is approximately 2,200 miles long, stretching from Cape Cod, through the southeast United States and through Mexico, ending with the Yucatan Peninsula.

The western border of the coastal plain is easily defined by a long series of mountain ranges, including the Appalachian Mountains and the Great Smoky Mountains. The eastern border isn't well defined as most of the plain is at or below sea level. Some define the east border to be the coastline. Others continue the plain through the coastal shelf that extends well beyond the coastline.

The geological theory is that the plain was formed as an underwater ridge when the ocean's covered the entire existing plain. This theory traces the underwater ridge to the initial separation of Pangea about 280 million years ago. See the articles on Pangea and plate tectonics for more information about this geological event.

A separate theory includes a meteor or small asteroid impact in the Gulf of Mexico that breaks up that particular part of the plain.

Currently, the coastal plain is very wet, including many rivers, marsh, and swampland. It is primarily used for agriculture.