Bloomsbury 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 792, a decrease of 78 from the 2010 census count of 870, which in turn reflected a decline of 16 from the 886 counted in the 2000 census. Bloomsbury was incorporated as a borough by an act of the New Jersey Legislature on March 30, 1905, from portions of Bethlehem Township.