Middlesex County is a county in the Commonwealth of Massachusetts. At the 2010 census, the population was 1,503,085, making it the twenty-third most populous county in the United States and the most populous county in 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.
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).
In 2010, the center of population of Massachusetts was located in Middlesex County, in the town of Natick.
, Middlesex County was tenth in the United States on the list of most millionaires per county.