Millstone is a borough in Somerset County, in the U.S. state of New Jersey. The borough was originally known as Somerset Courthouse and was the county seat. As of the 2020 United States census, the borough's population was 448, an increase of 30 (+7.2%) from the 2010 census count of 418, which in turn reflected an increase of 8 (+2.0%) from the 410 counted in the 2000 census.

Somerset County is a county located in the north-central part of the U.S. state of New Jersey. As of the 2020 census, the county was the state's 13th-most-populous county, with a population of 345,361, its highest decennial count ever and an increase of 21,917 (+6.8%) from the 2010 census count of 323,444. Somerset County constitutes part of the New York metropolitan area. Its county seat is Somerville. The most populous place in the county was Franklin Township, with 68,364 residents at the time of the 2020 census, while Hillsborough Township, with 55.00 square miles (142.4 km2), covered the largest total area of any municipality. The county is part of the Central Jersey region of the state.