Middlesex is a borough in Middlesex County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 13,635 reflecting a decline of 82 from the 13,717 counted in the 2000 Census, which had in turn increased by 662 from the 13,055 counted in the 1990 Census.Middlesex was incorporated as a borough by an act of the New Jersey Legislature on April 9, 1913, from portions of Piscataway Township, based on the results of a referendum held on May 6, 1913. The borough's name derives from Middlesex, England.