Winthrop is a town in Suffolk County, Massachusetts, United States. The population was 19,316 at the 2020 census. Winthrop is an ocean-side suburban community in Greater Boston situated at the north entrance to Boston Harbor, close to Logan International Airport. It is located on a peninsula, 1.6 square miles (4.2 km2) in area, connected to Revere by a narrow isthmus and to multiple portions of Boston by a bridge over the harbor inlet to the Belle Isle Marsh Reservation in the Neighborhood of East Boston, a shared border at the Boston Logan International Airport, and at Deer Island.

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.