Middlesex County is a county in the Commonwealth of Massachusetts, in the United States. As of 2014, the estimated population was 1,570,315, making it the twenty-third most populous county in the United States, and the most populous county in both Massachusetts and New England. For administrative purposes the county held two regions, Middlesex-North with its county seat in Lowell, and Middlesex-South with its county seat in Cambridge. Middlesex County is included in the Boston-Cambridge-Newton, MA-NH Metropolitan Statistical Area.