Middlesex County is a county located in the Commonwealth of Massachusetts, United States. As of the 2020 census, the population was 1,632,002, making it the most populous county in both Massachusetts and New England and the 22nd most populous county in the United States. Middlesex County is one of two U.S. counties to be amongst the top 25 counties with the highest household income and the 25 most populated counties. It is included in the Census Bureau's Boston–Cambridge–Newton, MA–NH Metropolitan Statistical Area. As part of the 2020 United States census, the Commonwealth's mean center of population for that year was geo-centered in Middlesex County, in the town of Natick.

Cambridge is a city in Middlesex County, Massachusetts, United States. It is a suburb in the Greater Boston metropolitan area, located 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.