Popponesset Island is an island and census-designated place (CDP) in the town of Mashpee in Barnstable County, Massachusetts, United States. The population was 220 at the 2010 census.
Popponesset Island is located in the southeastern part of the town of Mashpee. It lies in Popponesset Bay, an arm of Nantucket Sound, and is separated from the mainland of Cape Cod by the narrow Popponesset Creek. The CDP of New Seabury is to the west, across Popponesset Creek, and the Popponesset CDP is to the southwest. The town of Barnstable is to the northeast, across Popponesset Bay.
According to the United States Census Bureau, the Popponesset Island CDP has a total area of . of it is land, and of it (81.16%) is water.
During World War II, Popponesset Island was used for amphibious assault training.
List of Massachusetts locations by per capita income
At the 2000 census there were 39 people, 19 households, and