Boston [a] is the capital and most populous city in the Commonwealth of Massachusetts in the United States.The city serves as the cultural and financial center of New England, a region of the Northeastern United States.It has an area of 48.4 sq mi (125 km 2) [9] and a population of 675,647 as of the 2020 census, making it the third-largest city in the Northeastern United States after New York ...