Middlesex is a borough in Middlesex County, in the U.S. state of New Jersey. It is located within the Raritan Valley region. As of the 2020 United States census, the borough's population was 14,636, its highest decennial count and an increase of 1,001 from the 2010 census count of 13,635, which in turn reflected a decline of 82 from the 13,717 counted in the 2000 census.