Middlesex County is located in the north-central part of New Jersey, United States, 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 from the 2010 census count of 809,858, which in turn reflected an increase of 59,696 from the 750,162 counted in the 2000 census.