Somerville is a borough in Somerset County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 12,098, reflecting a decline of 325 (-2.6%) from the 12,423 counted in the 2000 Census, which had in turn increased by 791 (+6.8%) from the 11,632 counted in the 1990 Census. It is the county seat of Somerset County.