Winthrop is a city in Suffolk County, Massachusetts, United States. The population was 19,316 at the 2020 census. Winthrop is an ocean-side suburban town in Greater Boston situated at the north entrance to the Boston Harbor, geographically nearby to the Logan International Airport. It is located on a peninsula, 1.6 square miles (4.2 km2) in area, connected to the city of Revere, Massachusetts 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 line at the Boston Logan International Airport, and at Deer Island.