Rocky Hill is a borough in Somerset County, in the U.S. state of New Jersey. Named for the Rocky Hill Ridge, this historic village is nestled within the heart of the Raritan Valley region, located alongside the course of the Millstone River. As of the 2020 United States census, the borough's population was 743, an increase of 61 from the 2010 census count of 682, which in turn reflected an increase of 20 from the 662 counted in the 2000 census.