Millstone is a borough in Somerset County, New Jersey, United States. It was originally known as Somerset Courthouse and was the county seat. As of the 2010 United States Census, the borough's population was 418, reflecting an increase of 8 (+2.0%) from the 410 counted in the 2000 Census, which had in turn declined by 40 (-8.9%) from the 450 counted in the 1990 Census.
Millstone was incorporated as a borough by an act of the New Jersey Legislature on May 14, 1894, from portions of Hillsborough Township, based on the results of a referendum held that day. The borough was reincorporated on March 12, 1928. The borough was named for the Millstone River, whose name derives from an incident in which a millstone was dropped into it.
A historic district in Millstone, comprising 58 buildings, was added to the National Register of Historic Places in 1976. The borough possesses a military significance for 1700–1749, 1750–1799, 1850–1874.
New Jersey Monthly magazine ranked Millstone as its 7th best place to live in its 2008 rankings of the "Best Places To Live" in New Jersey.
Millstone, then called Somerset Courthouse, was the county seat of Somerset County from 1738 until the British burned it to the ground in 1779 during the American Revolutionary War. After the victory at Princeton on January 3, 1777, General George Washington headquartered at the Van Doren house, while the army camped nearby that night. The next day, they marched to Pluckemin on the way to their winter encampment at Morristown.
Millstone was briefly connected to the Pennsylvania Railroad when the Mercer and Somerset Railway was extended to the town in the 1870s and connected via a bridge across the Millstone River to the Pennsylvania Railroad's Millstone and New Brunswick Railroad, but that arrangement did not last into the 1880s. Remnants of the railroad bridge can still be seen.
According to the United States Census Bureau, the borough had a total area of 0.760 square miles (1.969 km2), including 0.738 square miles (1.911 km2) of land and 0.022 square miles (0.058 km2) of water (2.95%).
The borough borders Franklin Township and Hillsborough Township.
The 2010 United States Census counted 418 people, 162 households, and 117.936 families in the borough.