Middlesex County is a county located in the north-central part of the U.S. state of New Jersey, extending inland from the Raritan Valley region to the northern portion of the Jersey Shore. As of the 2020 United States census, the county was the state's third-most populous county with a population of 863,162, its highest decennial count ever and an increase of 53,304 (+6.6%) from the 2010 census count of 809,858, which in turn reflected an increase of 59,696 (8.0%) from the 750,162 counted in the 2000 census. Middlesex is part of the New York metropolitan area. Many communities within the county serve as commuter towns to and from New York City and other points north. The county is part of the Central Jersey region of the state.
The county is located in the middle of the Northeast megalopolis of the U.S. Its county seat is the city of New Brunswick, a center for the sciences, arts, and cultural activities, and the headquarters of the state's flagship academic institution, Rutgers University. The county's most populous place, with 107,588 residents as of the 2020 census, is Edison, while Monroe Township covers the largest area of any municipality, at 42.19 square miles (109.3 km). Since the 2010 census, the state's center of population is in East Brunswick; the center of population for New Jersey has been in Middlesex County since the 1900 census. Middlesex County hosts an extensive transportation network, including several rail stations along the heavily traveled Northeast Corridor Line of the New Jersey Transit commuter rail system, as well as the intersection of the New Jersey Turnpike and the Garden State Parkway, the state's two busiest motor vehicle roadways, in Woodbridge Township. Middlesex County calls itself The Greatest County in the Land.