Branchville is a borough in Sussex County, New Jersey, United States. As of the 2010 United States Census, the borough's population was 841, reflecting a decline of 4 from the 845 counted in the 2000 Census, which had in turn declined by 6 from the 851 counted in the 1990 Census. The borough is located in the northernmost region of Sussex County.Branchville was incorporated as a borough by an act of the New Jersey Legislature on March 9, 1898, from portions of Frankford Township. An additional portion of Frankford Township was annexed as of March 1, 1951. The borough was named for its site at a branch of the Paulins Kill.