South Bound Brook is a borough in Somerset County, in the U.S. state of New Jersey. As of the 2020 United States census, the borough's population was 4,863, an increase of 300 from the 2010 census count of 4,563, which in turn reflected an increase of 71 from the 4,492 counted in the 2000 census. What is now South Bound Brook was originally formed as a town within Franklin Township.