Middlesex County is a county located in north-central New Jersey, United States. As of the 2010 census, its population was 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. The center of population of the state of New Jersey is located in Middlesex County, in East Brunswick Township, just east of the New Jersey Turnpike. The 2000 Census showed that the county ranked 63rd in the United States among the highest-income counties by median household. The Bureau of Economic Analysis ranked the county as having the 143rd-highest per capita income of all 3,113 counties in the United States (and the 10th-highest in New Jersey) as of 2009.
The county was established as of March 7, 1683, as part of the Province of East Jersey and was partitioned as of October 31, 1693, into the townships of Piscataway, Perth Amboy and Woodbridge. Somerset County was established on May 14, 1688, from portions of Middlesex County.
The county's first court met in June 1683 in Piscataway, and held session at alternating sites over the next century in Perth Amboy, Piscataway and Woodbridge before relocating permanently to New Brunswick in 1778.