Middlesex County is a county located in north-central New Jersey, United States. As of the 2016 Census estimate, the county's population was 837,073, making it the state's second-most populous county, an increase of 3.4% from the 2010 United States Census, when its population was enumerated at 809,858, making it the second-most populous county in the state. Middlesex is part of the New York metropolitan area, and its county seat is New Brunswick.