South Bound Brook is a borough in Somerset County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 4,563, reflecting an increase of 71 from the 4,492 counted in the 2000 Census, which had in turn increased by 307 from the 4,185 counted in the 1990 Census.What is now South Bound Brook was originally formed as a town within Franklin Township. On March 16, 1869, the name of the community was changed to Bloomington, which lasted until May 29, 1891, when the name reverted to South Bound Brook town. South Bound Brook was incorporated as an independent borough by an act of the New Jersey Legislature on April 11, 1907, based on the results of a referendum held on May 1, 1907.