Marajó is an inhabited island, located at the mouth of the Amazon River in Brazil. With a land area of 40,100 km² (15,500 sq mi, see list of islands by size), it is the largest island to be surrounded by both fresh and salt water (its north-east coastline faces the Atlantic ocean). The city of Belém lies to the south, across the southern fork of the river's mouth.

In the 1918–1919 outbreak of the Spanish influenza, Marajó is the only major populated area to not have documented any cases of the illness.