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.