Mattapan (/mætəˈpæn/) is a neighborhood in Boston, Massachusetts. Historically a section of neighboring Dorchester, Mattapan became a part of Boston when Dorchester was annexed in 1870. As of the 2010 census, it had a population of 36,480, with the majority of its population immigrants. Like other neighborhoods of the late 19th and early 20th century, Mattapan developed, residentially and commercially, as the railroads and streetcars made downtown Boston increasingly accessible. Predominantly residential, Mattapan is a mix of public housing, small apartment buildings, single-family houses, and two- and three-family houses (known locally as "Three-Deckers" or "Triple-Deckers"). Blue Hill Avenue and Mattapan Square, where Blue Hill Avenue, River Street, and Cummins Highway meet, are the comm