Blairstown is a township in Warren County, in the U.S. state of New Jersey. As of the 2020 United States census, the township's population was 5,704, a decrease of 263 (−4.4%) from the 2010 census count of 5,967, which in turn reflected an increase of 220 (+3.8%) from the 5,747 counted in the 2000 census.