Plum Island, Massachusetts is an island located off the northeast coast of Massachusetts, north of Cape Ann. It is a barrier island approximately 11 miles (18 km) in length. The island is named for the wild beach plum shrubs that grow on its dunes. The island is located in parts of four municipalities in Essex County. From north to south they are the city of Newburyport, and the Towns of Newbury, Rowley, and Ipswich.
In the early colonial period, Plum Island was a source of salt hay.