Plum Island is a barrier island located off the northeastern coast of Massachusetts, north of Cape Ann, United States. It is approximately 11 miles (18 km) in length. The island is named for the wild beach plum shrubs that grow on its dunes, but is also famous for the purple sands at high tide, which derive their color from tiny crystals of pink pyrope garnet. It 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.

Prunus maritima, the beach plum, is a species of plum native to the East Coast of the United States. It is a choice wild edible and its few pests and salt tolerance make it a resilient fruit crop for degraded lands and urban soils.