South River is a borough in Middlesex County, in the U.S. state of New Jersey. As of the 2020 United States census, the borough's population was 16,118, an increase of 110 from the 2010 census count of 16,008, which in turn reflected an increase of 686 from the 15,322 counted in the 2000 census. What is now South River was originally formed as the town of Washington within East Brunswick Township on February 23, 1870.