Glen Gardner is a borough in Hunterdon County, in the U.S. state of New Jersey. As of the 2020 United States census, the borough's population was 1,682, a decrease of 22 (−1.3%) from the 2010 census count of 1,704, which in turn reflected a decline of 198 (−10.4%) from the 1,902 counted in the 2000 census.