Deer Island is a peninsula in Boston, Massachusetts. Since 1996, it has been part of the Boston Harbor Islands National Recreation Area. Although still an island by name, Deer Island has been connected to the mainland since the former Shirley Gut channel, which once separated the island from the town of Winthrop, was filled in by the 1938 New England hurricane. Today, Deer Island is the location of the Deer Island Waste Water Treatment Plant, whose 150-foot-tall (46 m) egg-like sludge digesters are major harbor landmarks.

The Boston Harbor Islands National Recreation Area is a national recreation area situated among the islands of Boston Harbor of Boston, Massachusetts. The area is made up of a collection of islands, together with a former island and a peninsula, many of which are open for public recreation and some of which are very small and best suited for wildlife. The area is run by the Boston Harbor Islands Partnership. It includes the Boston Harbor Islands State Park, managed by the Commonwealth of Massachusetts. Twenty-one of the 34 islands in the area are also included in the Boston Harbor Islands Archeological District.