Mattapan is a neighborhood in Boston, Massachusetts. Historically a section of neighboring Dorchester, Mattapan became a part of Boston when Dorchester was annexed in 1870. Mattapan is the origina…Mattapan is a neighborhood in Boston, Massachusetts. Historically a section of neighboring Dorchester, Mattapan became a part of Boston when Dorchester was annexed in 1870. Mattapan is the original Native American name for the Dorchester area, possibly meaning "a place to sit." As of the 2010 census, it had a population of 36,480, with the majority of its population immigrants.