Somerville is a borough in Somerset County, New Jersey, United States. As of the United States 2000 Census, the borough population was 12,423. It is the county seat of Somerset County. Somerville was originally formed as a Town on March 25, 1863, within a portion of Bridgewater Township. Somerville was incorporated as a borough by an Act of the New Jersey Legislature on April 16, 1909, based on the results of a referendum held on May 4, 1909, at which point it was fully set off from Bridgewater Township.