The ecoregion covers and area of 11,300 square kilometers (4,400 sq mi) on the Atlantic coast of Maranhão and eastern Pará states. The ecoregion is divisible into eastern and western parts. The western part extends from near Belém on the Pará River along the coast of eastern Pará and western Maranhão states to the Baía de São Marcos. Here the coastline is made up of hundreds of islands and mudflats, made up of fine silt deposited by the Amazon River.
The eastern part extends from the Baía de São Marcos along the eastern coast of Maranhão to the mouth of the Parnaíba River. Here the coast is dominated by extensive sand dunes, interspersed with pockets of mangroves in bays and river mouths.