Cambridge ( KAYM-brij) is a city in Middlesex County, Massachusetts, United States. It is located in the Greater Boston metropolitan area directly across the Charles River from Boston. The city's population as of the 2020 U.S. census was 118,403, making it the most populous city in the county, the fourth-largest in Massachusetts behind Boston, Worcester, and Springfield, and ninth-most populous in New England. The city was named in honor of the University of Cambridge in Cambridge, England, which was an important center of the Puritan theology that was embraced by the town's founders.