Cambridge is a city in Middlesex County, Massachusetts, United States. As part of the Boston metropolitan area, the city's population of the 2020 U.S. census was 118,403, making it the fourth …Cambridge is a city in Middlesex County, Massachusetts, United States. As part of the Boston metropolitan area, the city's population of the 2020 U.S. census was 118,403, making it the fourth most populous city in the state, behind Boston, Worcester, and Springfield. It is one of two de jure county seats of Middlesex County, although the county's executive government was abolished in 1997. Situated directly north of Boston, across the Charles River, it was named in honor of the University of Cambridge in England, once also an important center of the Puritan theology embraced by the town's founders.