Middlesex County, in the Commonwealth of Massachusetts, is the twenty-third most populous county in the United States and the most populous county in New England. As of the 2010 census, the population was 1,503,085, making it more populous than any other New England state except for Connecticut. The center of population of Massachusetts is in Natick, Middlesex County.
For administrative purposes the county held two regions, Middlesex-North with county seat in Lowell, and Middlesex-South with county seat in Cambridge. The county government was abolished in 1997, but the county boundaries continue to describe a state district for court jurisdictions and for other administrative purposes, such as an indicator for elections. Massachusetts counties also define locations for National Weather Service weather alerts (such as severe thunderstorm warnings).
, Middlesex County was tenth in the United States on the list of most millionaires per county.