Marsh Island is an uninhabited low-lying marshy island in Iberia Parish, south coastal Louisiana, lying between Vermilion Bay and the Gulf of Mexico.

It is bordered on the south by the Gulf of Mexico and is separated from mainland Louisiana by East Cote Blanche Bay to its east, West Cote Blanche Bay to its north, Vermilion Bay to its northwest, and Southwest Pass to its west.

Other animals which inhabit the island include alligators, ducks and other waterfowl, crabs, and shrimp.

The island has a land area of 258.95 square kilometers (99.98 sq mi) and had zero residents as of the 2000 census.

The island has many named features that includes areas, bayous, and lakes.